The aid on the ship was symbolic. This was a protest action to call attention to Israel's continued blockade of food and other aid into Gaza.

Their goal was to get attention, to force the media and public to start talking about it, to force western governments to pretend to care about it, ratchet up the international pressure by another notch. They have done everything they need to be successful at that. Now that they've been seized, it's our jobs to do what we can to make the rest of that happen.

I don't think they believed they would be kidnapped and detained in international waters, to be fair.

Well sure, they knew this was a likely outcome - because they know the Israel government is batshit. The purpose of the mission was aid and activism. They definitely knew that arrest was likely, but that's not why the did it. They didn't want to be detained. They just want to help the people of Gaza in whatever way is within their power. This is how they chose to do it.

It may not be perfect, but it's definitely more effort and effect than doing nothing. And the more people who get on board with some actions, the more likely they are to succeed.