As a motorcyclist, I am wary of boats on trailers, you never know when a wheel will come off, or what will pop out. I watched a life jacket float around in one for a few miles, then it popped up, the wind caught it and it's strap brought a 6 foot wooden oar with it. One of my friends was killed by another negligent boater on the road.
Wheel bearings get warm when you use them. When you dunk them in cool water, everything contracts and it sucks water in. The water pits the bearings, and then they continue to get worse like a pothole in the road. This happened to yours, replace them all. This can be avoided by installing the grease seals backwards, so grease can escape but water won't get in, and replacing the caps with "Bearing buddys", or "bullseye" caps. They have a Zerk fitting and hold the grease under spring pressure, and any excess is pushed out behind the wheel . I have found the Bullseye leaks less grease film. They may make a small mess, but being stuck along the roadway with a ruined spindle, or causing a wreck, is far worse. Use a waterproof grease, and never mix it with another type.