You will likely be cutting it very close and will more than likely miss the ship. There is an orientation period that lasts 20 minutes or so and also they never start on time. My advice, do not do the excusrion at 3:30. Of course, do as you want, but one of two things will happen (I now because I saw it happen when I was there). First, you will have to leave the swim before the better parts are started, or secondly, you will miss the ship.