1) Pump the hub full of grease often? If so, how do you tell when it's full? If you can't tell when it's full, you'll eventually blow the rear seal, which stands a good chance of allowing water intrusion. Of course, this only applies to hubs fitted with zerk fittings (hard to overfill if you're packing by hand.)
2) Arrive at the ramp, and immediately dunk the trailer? Bearing surfaces heat up as you're running down the highway, causing the grease to expand somewhat. Immediate dunking causes rapid cooling, which can create a slight vacuum within the hub. That vacuum is relieved by whatever is around the bearing; in that case, seawater. You should allow a bit of time in the lot for things to cool down prior to immersing the hubs.
3) Occasionally pull whatever you have for a bearing dust cover and take a look at the bearings (or, ideally, the grease, as you won't see the bearings)? Grease color will be a fair indicator: If it's milky, you've had water intrusion, and will have a problem. If it smells burnt, you've already got a problem. If there's no grease, it's likely that you have a problem.
4) Also occasionally (once every year or two) disassemble the hub, and wash and inspect all the bits and pieces, repack with grease, and reassemble? Probably should.
5) Check hub temperatures while trailering? An occasional stop to verify temps is a wise thing to do. On my trailer, after an hour of running, the hub will be just warm to the touch. If it's hot, you've potentially got a problem. If the grease is smoking, there's definitely a problem. Some people use an infrared temp guage to scope this out. Probably cleaner than my way.
FWIW, I don't think that hosing off the trailer is going to do anything for the bearings. They're too sealed up for a rinse to do any good.
Usually bearings fail due to water intrusion (and either the resulting salt encrustation or corrosion, either of which causes more friction, which in turn overheats the grease), or from maladjustment (too tight creates a ton more friction than having it set just right - this burns off the grease, leading to failure.) Too loose allows slop between the pieces, which will eventually cause flat spots on the bearing rollers, leading to failure.
If you need info on how these things all go together, there are a number of resources on-line. Here's a few, though: