While boating this weekend, I noticed many trailers in the parking lot had external grease zerks in the center of the hub, do you? If so I would imagine you could shoot some new marine grease in after every submersion in the lake to force the water out?
If not, you would have to take it all apart obviously and repack. I personally would want to do it at least once a year, but not any more frequently due to the labor involved. While on there check for bearing wear and replace as necessary?