My race boat trailer has double lip seals. Pulled it about 35000 miles before I repacked the bearings. Being as it's a race trailer, it never gets put in the water with hot hubs. That may make a difference.
if you put 'em in the water when they're hot after a long tow, the rapid cooling will create negative pressure in the bearing housing and pull water in. if you're going to drop something in right after towing awhile, hit the zerks/buddys with a couple shots right before you put it in the water. that will help keep the water out.