The reason you don’t see WDHs used with boat towing is because the trailer tongue hitch weight is well within the guidelines of the 10%-12% of gross towed weight.
You’ll also notice the majority of boaters with boats weighing in excess of 2000 lbs are driving heavy vehicles.
Most boats (including cabin cruisers) have the engines (heaviest component) either on the stern or are located amidships.
These truck/trailer combos are also equipped with brake controllers (Same ones used as those towing travel trailers).
Those heavy duty trucks (Dualies) that tow up to mid-sized cruisers use a WDH such as the Equal-i-zer or a Hensley which stop sway BEFORE it begins and as such do not need an accompanying sway bar.
Boat design does not have the same wind drag characteristics as a travel trailer.
Other WDHs need sway bars as they don’t prevent sway from the start. This is by design.
I use a Reese WDH with a sway bar when I tow my travel trailer, but if I decided to upgrade the set up I have I would go for the more expensive (and much better designed) Equal-i-zer. The Hensley runs about $3-4k.
Like the rv’ers say, when it comes to a tow vehicle,"bigger IS better’.