Yes, boat trailers are a necessary evil, but assuming you do not dry stack, one we have to put up with because alternative systems are expensive (search on Bedajim for exotic trailers!) Anyway, with proper maintenance a 'normal' british trailer will be reliable for a season. So, to assure reliable service, try to service the trailer each year.
Searches on Ribnet will give you the does and don'ts, generally open taper bearings (preferably with bearing savers) as opposed to the sealed variety, are preferred, and keep on top of brake maintenance (if fitted). It is often brakes that cause bearing failure by binding, getting the hub assembly hot, and causing the grease to abandon ship.
After each dip flush out the brake drums as best you can - there are after market flushing kits you can fit. Some even suggest that brake drum flushing is not necessary - I subscribe to the flush it if you can view, but form your own opinions! Other than this, a general hose down to get the salt off, and common sense check over of the hitch and bolt tightness is about all you can do.
Dust creation specialist