If you pull that rubber insert out I think you will find a zerk fitting and that those are Dexter EZ-Lube axle spindles. They should be given a few pumps (of matching grease) every time you pull that out of the water for a trip. If it's red probably Lucas red-n-tacky, if blue, probably Lucas marine grease.
The inner cover is probably weeping a little around the hub, but pull the wheel and check everything.
Don't know how often it's in/out of the water but bearings should be periodically repacked depending on time/use, especially salt water. Is whomever is winterizing for you also doing bearing maintenance?
I carry extra sets of bearings for my trailers, but stay on top of maintenance. Never fun trying to change bearings on the side of the road, and super not fun not finding a local parts store with a set of bearings.