As has been said already, dyslexia varies significantly in severity. I dealt with this issue an untold number of times when I was a classroom teacher. Thinking of the most seriously afflicted I met, I don't think they would have as much trouble with the diving as with learning the academic stuff required for getting certified. If they are severe enough to have an IEP or even a 504 plan clearly identifying the problem, then they would qualify to have the test read to them and then give the answers orally.
The most severe student I ever had was totally misdiagnosed throughout his life. Tests indicated that he had the reading and writing ability of a 2nd grader, so his special education teachers gave him 2nd grade level materials to read and write about his whole life. Through a maneuver that does not need to be explained here, he eventually escaped from that (quite literally) got into my 12th grade English class, which he passed with flying colors by using a screen reading program that read the literature to him and a screen writing program that converted his speech to writing. His ability to comprehend literature and respond to it on an intellectual level was comparable to the top third of the students I ever had in an Advanced Placement class, but if you handed him an essay he had just written himself using the screen writing program, he could not read it to you word for word--although he could still explain the concepts.
Even someone as severe as that can handle scuba, IMO. It is a matter of having the proper gauges. As squid509 said, if you are familiar with the analog gauges and know what it means when the needle is pointing the right way, or if you know what it means when the computer bar is moving to the yellow zone, it is easy to convert that information to proper action.
The vast majority of dyslexics are nowhere near that severe and can handle scuba with no real issues at all.
By the way, there are people diving with all sorts of handicapping conditions. There are even blind divers. The key is to find the appropriate way of dealing with the inherent problems.
