Well, metal does rust. but the saltwater is what will really make it rust quicker. with that said I would wait until the exhaust starts leaking. then I would replace it. It is nothing to get stressed over.
In my younger years I would be driving on the beach for fishing 3-4 days a week with my 4X4’s. when I got home I would rinse the underneath of the vehicle and the body with fresh water to remove the salt and sand. you could get a hose and sprinkler and put it under your vehicle to rinse it, just move it to get the whole underneath of vehicle.
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Maybe you actually take too many short trips, don’t know. But that’s not the cause of the external rust. I expect it is the occasional sea water splash, mostly just the salt in the air. Short trips for the most part causes the exhaust system to rust from the inside out, not outside in. I guess if I had that sort of problem I’d be asking some of the well-recommended shops around town for quotes on installing a new exhaust system. Proactive repair, b/c your engine likely won’t run well after it develops the first exhaust leak. Ask the shops if it might be possible to keep the existing catalytic converter. That would save you some $$ on the parts cost. Also ask if they’d recommend replacing the existing system with components made of materials that are more rust resistant; i.e. pay more now, but it lasts longer.
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The mechanic is just someone I went to to get a second opinion. I get all the work done at the Subaru dealer. The Subaru dealer is telling me it’s because of where I live. I don’t believe it. I get my oil changed at least twice a year. Nobody has said anything about sludge.
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I live in an apartment so I go to the car wash and that has a rinse for the undercarriage. I avoid the salt water in the roads now.
But the damage is already done. Good practice for the next time. That picture above is nothing to be concerned about at all. Minor surface rust and very normal. The people telling you to dump the car because of that exhaust rust are nuts. As most have already told you, stop worrying, drive until it leaks and replace the entire exhaust front to back. Don’t allow salt water to touch it if at all possible. Occasionally take it for longer drives and it’s likely to outlast your desire for the car…
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The response depends on the question asked: “Am I in danger?” No.
“Can you provide an estimate to replace every rusted part?” Don’t waste my time, find a different car.
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Certainly interesting . Does Suburu not use stainless exhausts ? My 2008 looks better than that and is on the original exhaust and they salt the $#$#$#@$ out of the roads here .
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The exhaust on cars have been made out of stainless steel for decades now. There used to be muffler shops everywhere since the exhaust would need to be replaced on the older cars. The exhaust pipes are made out of some 400 series stainless steel which is sufficient to last the life of the car. 300 series is better and is non magnetic. Some use thick flanges which are not stainless.
Notice in the picture that the flange has the typical minor surface corrosion of 400 series stainless, but the pipe looks like a car from the 70s. Have they moved away from using stainless exhaust? They’ve stopped putting the preservatives in the rubber used in car tires for the last two decades, so tires have to be replaced within 10 years even if not worn now. I forgot about the stainless exhaust issue.
Check out other cars of the same model and year to see if this is happening too. Maybe the exhaust was already replaced by a muffler shop. They usually won’t use stainless unless asked to. If it’s not happening to other Subarus, get a replacement from a junk yard that doesn’t trash the whole exhaust when they take the catalytic converter off. If the rust is happening to other models, look in to the cost of a replacement and decide if you want to keep it or sell it.
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The difference is the temperature. I’m betting it’s not 80 degrees and sunny when they salt the roads 
As an example, try putting that salt slush car in a heated garage every day after driving through the slush. The bottom will look like this exhaust, trust me on that one.
Not too long ago, I was working at a facility right on the shoreline. I used to go striper fishing during lunch The Operations people were always trying to reduce costs on production products, sometimes at the expense of performance or reliability due to their ignorance of the importance of certain aspects of the design. One time, they wanted to remove a protective coating on a stainless steel enclosure. Mind you, this product was used on marine buoys. The Mechanical Engineers were sick of battling with these situations so one of them took the FA enclosure, tied a rope around it and tossed it into the water. He came back a week later and fished it out. It was very corroded already. It doesn’t take long for unprotected metal to degrade in seawater. Stainless is not impervious. There are many formulations of stainless as noted but the most resistant is not used for the tubing in exhaust…
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I would find a few other local Crosstrek drivers to see if they are having the same problem. Something is suspicious. The Suby dealer should be quite familiar with the problem. Did someone trade out your SS system and replace it with junk at some point? Was your catalytic converter ever stolen? Or, did Subaru run out of SS systems when building your car?
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I agree and rusting can be minimized by passivating the surface. As I’m sure you know @TwinTurbo, passivation dissolves the free iron and leaves a fully oxidized surface, comprised mostly of chromium and nickel oxides. This barrier coating won’t oxidize further. The coating can be enhanced by using a hot nitric acid passivation. Passivation costs more and some manufacturers might minimize the treatment to save on costs.
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I’ll just note that in Tampa, salt and moisture will be in the air. No need to drive through salt water to get the effect of salt on metal parts. The lack of rust on the under carriage may be related to the body rust proofing on new cars now. A little frame rust on heavy metal parts is just surface rust. Rust is natures way of reclaiming the elements and can’t be stopped.
All that has been said though I suspect is falling on deaf ears to someone fearful of any rust. There is no other way to overcome this irrational fear than to trade for a new car and start over.
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+1
In the early 1990s I was on an extended business trip near Lompoc, CA, a small Pacific Coast town. I met a guy with a pristine mid-1960s Mustang. I commented about the Central/Southern California weather and how well cars did in it. He disagreed and said that he bought the car while working at a Federal prison in West Texas a couple years earlier. He was concerned that the salt air might have an adverse effect on his Mustang.
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I think you have to be pretty close to the beach for salt-air to have much of an effect on a car. One friend grows avocados for a living, lives near the coast, about 2.5 miles distant & 500 feet above the beach, no salt air corrosion problems. Biggest problem for him is people driving by, deciding to stop & sample a few of his tree’s avocados.
Florida is different, I live about 10 miles from the beach, one mile from the intracoastal waterway, my 1986 California car started showing surface rust within six months of moving to Florida.
We have onshore sea breezes from both sides of the peninsula.
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I looked underneath Subaru’s when I had my car serviced at the dealership recently. I looked at new and old cars that were parked for service. Even some cars from up north and nothing like the rust have. No the catalytic converter has never been stolen.
I am voting for the theory that someone switched out their old system for your nice new one at some point. For those with the theory that the car did not get hot enough, I would say that the system would have less rust near the front of the exhaust system and would probably have the muffler rusting out from the inside.
See if you can find a system with a lifetime warranty.
I’m sticking with the cause being what the OP posted further up…
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+1 to Twin Turbo’s comments. And, although the OP has learned that it is not a good idea to drive through salt water, the damage has already been done.