Most of the half-day or shorter tours have multiple departures. You will receive a full schedule in your information package containing the descriptions of the tours. Princess sends the package via your travel agent about 60 days before your cruise.
If you are looking at the cruise in the "semisubmersible craft" (which actually is a boat that does not sink at all, but which has a compartment below the waterline with windows in the hull), I recommend the "Sea and See" tour instead. The "Sea and See" tour combines the cruise in the "semisubmersible craft" with a land tour that visits both Hell and the world's only sea turtle farm, both of which are fascinating stops.