My question to you is: Where you plan to take the Atlantis Submarine. Some are out of commission because of the hurricanes.
As far as the submarines are concerned, yes, the Atlantis submarines I have ridden and seen require walking backwards down a ladder into the hull. Once you are inside, you walk down a narrow isle and sit on seats facing porthholes. You will experience pressure as the sub dives. I would not recommend it to pregnant women or people with mobility issues.
First, you may will have to travel by boat to the sub launch area. That can be easy enough and then you have to transfer to the sub. Sure footing is a must.
Second, negotiating stairs backwards is difficult for even the most agile person. Those with limited strength will be especially vulnerable.
Third, the pressure from the dive can decrease the availability to expand the lungs, if somewhat minimal. It can also increase anxiety. Under water motion can also enhance motion sickness.
Maybe a better excursion would be a dolphin interaction. It is less strenous and will create a wonderous experience to last a lifetime.
Fair winds and calm seas.
~endkaos