We did the Seaworld Explorer in Curacao. From the surface, it is a flat decked boat, looks a lot like a barge. There are two hatches, fore and aft (front and back). You walk into the rear hatch and down the steps, into the hull. The hull is a deep V shape, maybe 7foot of height, so no stooping required. The the bottom of the hull is only about 4 feet across and there are metal seats folded up against the side. You go in two by tow (like Noah) and fill it front to back. When everyone is in, you put the seat down and sit. There are huge windows along the entire length, both sides. You are roughly 8 to 10 feet below the surface. If kids get scared or someone panics, they can get out, but it is not easy, you need to climb over a few people to do so. There is no room to walk around.
That said it was pretty cool. Although I think next time my wife wants to do the Altantis submarine. I like to actually be in the water (diving or snorkeling) so I will probalbly skip it.
The underwater life in Aruba is some of the best, so if you are not into getting wet yourself, I would suggest this or the submarine.
We did the Seaworld Explorer as part of a larger Island tour in Aruba last December. The previous post did a good job of explaining the inside of the vessel. However, if you do this don't sit in the front third on either side. The bubbles from the propellers blocks some of the view. Yes their are propellers on the front along with the back of the boat. Also be aware you don't actually submerge. You sit below the water line. I was somewhat disappointed only because we had done the Atlantis Submarine in Grand Cayman in 1999.