I think a good wash down of Boat engine and Trailer with fresh water ...or even better a Powerash is an essential part of the maintenance routine,the habit also gives you time for a close visual inspection of the component parts.
Personally I wash with a Carwash shampoo,with a little "Salt off" thrown in,then a good stiff rinse!
With a decent power washer you get in all nooks and crannies around the engine saddle/hydraulics ect,as well as the trailer Hubs..especially if you a flushing kit fitted.
I have seen the difference it makes when it is NOT done on a regular basis...and the results are not pretty!
...not to mention costly to put right...and potentially dangerous in the long run.