Warm wheel bearings will suck water into the bearing area every time to you launch your boat and the hubs go into cold water, seals or not. The good synthetic grease will protect your bearings, the old soap based greases will dissolve in the water. You really can not get by with water there every time you launch. The best move to make now is to jack up each wheel at the axle and spin the tire. Any noise or growling present means to change and lube your bearing now, not on the road. If you see your wheels wobbling going down the road in your rear view mirror, you are in a bad situation.