Deal with it.
Im not going on anybodies boat uninvited.
Can you imagine, in this day and age what might happen?
God forbid, some thing goes wrong with his boat, well so and so neighbor saw you on board just the other day. Your inviting and opening your self up for trouble. I know, its very sad, but it is the times we live in.
Maybe leave a friendly note for the next time it happens or talk with the harbormaster, but I don't believe its your place to go aboard another persons property no matter how much of a nuisence or inconvenience it causes you.
Courtney is My Hero
If a man is to be obsessed by something, I suppose a boat is as good as anything, perhaps a bit better than most - E.B. White