As the boat is approaching the island, Bond and Severine are at the front of the boat, looking at the island. They are NOT tied up yet. 

Severine says something like "we still could turn around" and Bond knowingly says something like "no, I don't think so" -- just as the shot widens to show the ship's crew slowly approaching Bond and Severine with their guns drawn.

So **the crew took Bond and Severine prisoner just before reaching the island.**