I have a hard time accepting any kind of hubs on a boat trailer as being maintenance free. Just the fact they they're being submerged at least twice every trip you make tells me that there will always be a chance that some water might get into the hubs/bearings.
And as someone else mentioned, when you drive a considerable distance and everything heats up, dunking them into water that's cooler than the temperature of that assembly is going to, by nature, cause them to contract and suck water into them. Even if it's a tiny amount, it's still getting in there.