There are several reasons boat trailers have so many wheel bearing problems:

One, they tend to just sit for long periods. Don't underestimate this one folks. Bearings are ment to spin. Just sitting in one place with weight on will cause flat spots to develope. Jack up your trailer and set it on blocks for the winter. Or better yet, take it out to the beach and go sailing once in a while .

Two, trailer manufacturer's use the smallest, cheapest bearings they can get away with. The industry is price driven, everybody shops prices and buys the cheapest.

Three, high wheel RPM. Get the largest diameter wheels and
tires that will fit under the fender if you tow long distances. Circumference is Pi x Radius squared. Do the math and find out how much harder your trailer bearings are working compared to your car wheel bearings. Again, your trailer probably came equipped with the smallest, cheapest ones the dealer could find.

Four: water getting into the bearings. Usually this happens
when you submerge the hubs for boat launching. Initially this doesn't cause much harm. But when the bearing heats up
on the road this moisture can be converted to steam, effectively "steam cleaning" the grease off of the hottest part of the bearing. Bearing Buddys help this problem. But
be carefull when packing the hubs with a pneumatic grease gun. I have seen quite a few inner grease seals blown off by overzealous grease pumping, making it easier for water to get in.

As Forest Gump says: "That's all I have to say about that"

Good luck
The Grease Monkey