Inside the twelve mile limit, Israel as the occupying power would have been within its rights to intercept the flotilla, outside of it however, it was an act of piracy on the high seas. The passengers were well within their rights to repel borders and no legal action against them is admissible in any honest court. There is no defence of self defence available to the marines because it was an act of piracy to begin with.
French tourist arrested for waving Israeli flag on Temple Mount (Haaretz)
from the article: Straight into the trap