We did the submarine excursion in April and loved it. You take a boat out about 2 - 3 miles to the sub and we were under water about 30 minutes or so. We went down about 75 feet and cruised near the ocean floor, where we saw several sharks and many other smaller fish. We also a saw a very large sting-ray, that was at least 5 - 6 feet across. We were the last family to board and got to sit at the front of the sub, so we could see out the huge window in front of the pilot as well as the side windows at our seats. My DS9 had a great time and we would recommend it.
If you want to do it, I would try to book it now. It is pretty popular and I think the sub only holds around 40 people, so it sells out quickly.