"Sealed" bearings are a buzz word. Most often this means the bearings are "cartidge" bearings with little rubber seals on the sides. Unfortunatley, water can get past these seals just the same as the rubber seal on a loose ball bearing component. When a "sealed cartridge bearing" get contaminated it's a lot more difficult to service the bearings. For this reason I favor loose ball type hubs since they are easy to service. Hubs like this should be serviced once a year or so.