I doubt whether they have any legal right to assault anybody - what power is there for somebody to self-declare a total exclusion zone round a civilian ship?! Airports and quaysides are different because it is privately owned property but the vessel owners don't own the wet bit, last time I looked it was Ma'am's.
I have been fairly close to large cruise ships here (~200m or so) with no objections, and closer than that to the Royal Navy vessels that tootle past on the odd occasions, nobody gets stroppy unless you do something stupid and I always stay well out of the way of the cruise ship tenders full of passengers.
I wouldn't go thundering towards the ship with a turban on my head, and I wouldn't make it look like I was trying to get aboard them, but if somebody had a go at me with a water cannon just for going to take a look I'd be right royally p***ed off.
Even if they did wish (or were allowed) to discourage people from approaching they should first challenge the vessel approaching with a loud hailer or something, at most a warning shot with a water cannon, and the only circumstances they could possibly justify taking direct action would be if somebody ignored this and was clearly trying to get close to the ship and I'm not even sure if it would be legal then.
Unless we all want to behave like the Israelis....