[LIforrelaxin]

First don't touch what isn't yours with out talking to your neighbor first.

I would offer to help relocate the swim raft back to in front of the right home. If they give you to much flack call the Marine Patrol, and I am sure they will talk to the home owner, who identification should be on the raft. If identification isn't on the raft then the Marine Patrol may just take the raft away.

Second, find out your town noise ordinance on noise, and "quite hours".... and then talk to your neighbor about it when they are too loud to late at night.
Once you know they are educated on the noise ordinances, then call the cops if they keep it up to often.