Amber red cross over pipe

During a recent flood in Port st lucie Florida the drive through the flooding I noticed that the cross over pipe was and amber redd and the engine was hesitating as well as the check engine light coming on. the codes indicated that there was a mis-fire in spark plugs 4 and 6. I replaced the plugs and the boots and when i started the engine there was a sound like marbles rattling and and the car won’t drive. What could be the cause? What should be my concern?The vehicle is a 2002 Mazda Tribute V6 engine.

My wild guess is a plugged exhaust, probably at the cat converter(s). Cat converters operate at very high temperatures and have a ceramic honeycomb in them that may have gotten thermal shocked and broke apart when you submerged them in flood water while hot. If so, that could plug the exhaust stream.

Mind you, this is just a guess.