Can a sheriff come in to your property without a warrant?

Not without a warrant.

If the place where your boats are kept is considered in your curtilage, an officer needs a warrant to go and inspect the boats. However, if the boats are stored on a section of your property that would be considered open to the public, the officers do not need a warrant. You should talk with a local attorney to see if your rights have been violated and whether you have a recourse.

Depends upon whether it is considered a search and the cop is looking for evidence of a criminal offense. Failure to license you boat could be considered criminal, or civil offense. If criminal there must be some sort of penalty for failing to license and in that case entry without a warrant would result in the suppression of any evidence recovered, in this case the cops observations.