Another problem that occurs with grease filled hubs is rust buildup on the surface the seal rides on. This ruins the seal. As mentioned before as the heat builds in the hub the air/grease expands, when you back into the water over the hub this quickly cools the hub sucking water inside. For all the time it takes before you use your trailer for first time pull the hub off and take a look. If necessary to replace bearing make sure the seal surface is clean and smooth. Also a good idea to coat grease outside the seal to help prevent seal surface rust. The oil filled hubs excel but they too need to be maintained. also make sure your tire pressures are good. You cant tell by visual check. USE THE GUAGE. Look at your tire sidewalls at the same time. Trailer tires are notorious for cracking. Better to check them before you head out. How many times have you seen trailers at the side of the road with a tire or a hub off?