Assemble a first-aid kit. In a surplus ammo can, place spare bearings, seals, a seal puller, marine-grade bearing grease, mineral spirits or kerosene, a small notepad and pencil, and this page of FIELD & STREAM. Stow it in your boat.
Disassemble. Loosen the lug nuts, jack up the trailer, and pull the wheel from the hub. Take off the grease cover or Bearing Buddy-type cap. Release the cotter pin from the spindle nut, then remove the nut and washer, and the entire hub should slip off the axle. There are two sets of bearings: one on the inside and one on the outside. Remove them. Tap out the grease seal. If you' re unfamiliar with the process, or if your hub is configured differently (some are), sketch out which parts come off in which order with the notepad and pencil.
Clean. Clean all the old grease from the bearings, hub, and axle spindle. Wipe away what you can with paper towels, then use mineral spirits or kerosene. Inspect the parts carefully for rust spots, cracks, or bluish staining that indicates overheating. Replace if necessary.
Repack. Place a glob of marine bearing grease in the palm of one hand and hold the bearing with the fingers of the other. Work the grease into the bearing cage, turning it to lubricate each bearing. Put a thin coating onto the spindle and inside of the hub.
Reassemble. To replace the grease seal, position it in the grease bore, place a piece of wood across it, and tap the wood lightly with a hammer. Then replace all the other items in order.