Greta has no official political power, and somehow she expected to… What?

If you are asking this question honestly, you should read From Dictatorship to Democracy. Basic TL;DR, every power structure that has ever existed in human history is built out of a tower of humans all, more or less, obeying the power structure. If the people involved stop cooperating, the whole edifice collapses. It can happen literally in weeks or days. By far the most effective way to undermine support for a monstrous regime is to destroy the base of people who are supporting it: In this case, the international community of political leaders that prop up Israel (or at least don't condemn it in any way that matters), and the people inside Israel who actually run the machine. By far the most effective way of doing that is to expose the brutal nature of the system, and turn people against the idea of cooperating with it.

The stated goal of the flotilla organizers was to open a humanitarian corridor. To keep sending clearly unarmed civilian vessels, which of course will several times get intercepted, until the Israeli narrative is no longer sustainable and they decide to let humanitarian aid into Gaza again. I have no idea whether they would add to that the goal of laying bare what the nature of Israel's brutality is, by deliberately exposing Western people to it in addition to "Arabs/Hamas/terrorists/whatever" being the only targets. But I wouldn't automatically assume that isn't part of the goal. And, if that is part of what they're trying to do, it is probably one of the most effective ways of ending the genocide that is available to anybody.