MY sisters and a nephew, who cruised with me this past February, took both the semi submersible and the 100 foot deep sub rides at Cozumel.
From the video, the semisub looks a lot like the Capt Nemo ride at Disney World. A somewhat elongated, steep stairway to get below the waterline. I have seen similar stairways on preserved WWII era submarines. Some folks might have a problem with that. However, there might be some provisions--perhaps disabled board first.
You might try to find the website for the operators of the subs and ask those questions directly. Or you might want to consider a glass-bottom boat as an alternative. The water is almost crystal clear, so the view would still be good. You just wouldn't have the bragging rights you would after a sub ride.