You're right...it really depends on the nature of the owner, and what the
intent is of the person boarding.

In Lake Cumberland, there are literal pirates. They paddle their power boats
up to unsuspecting house-boaters at night, and do a type of home invasion,
stealing everything from clothing, to electronics, and even furnishings.
They typically disable the boat before their departure.

I know that's somewhat extreme, but I do know house boaters there, that have
armed themselves, and are more than willing to shot first, and ask questions
later.

But back on track, generally I would NEVER board someone else's boat without
permission, but I would go out of my way in an emergency. For example, I'm
sitting in the marina, and a boat pull into the next slip...ties off, and
the folks go ashore. After they are out of sight, I notice their boat
filling water. You bet I'm jumping onboard to man the pumps, and try to stop
the leakage.

But outside of a situation like that, I say stay off.
--
-Netsock

"It's just about going fast...that's all..."
http://home.insight.rr.com/cgreen/