It appears most likely that there will be a standoff with the Israeli authorities tomorrow and that the activists will be arrested rather than be allowed into Gaza.
Although Israel withdrew its soldiers and settlers in 2005, it still controls Gaza's air space and sea space, as well as nearly all the border crossings.
Until a recent ceasefire with Hamas - the Islamist group which won Palestinian elections more than two years ago and which now fully controls Gaza - the Israeli military was mounting regular incursions into the territory, saying it wanted to stop rocket fire into southern Israel. It has also imposed a tight economic blockade aimed at weakening Hamas.
In a statement issued as they departed today, the activists said they would lodge a legal protest against any attempt by the Israelis to arrest them.
"If Israel chooses to forcibly stop and search our ships, we will not forcibly resist," they said in a statement.
"If we are arrested and brought to Israel, we will protest and prosecute our kidnapping in the appropriate forums ... It is our purpose to show the power that ordinary citizens of the world have when they organise together to stand against injustice."