Divers with disabilities are just divers. The only change is access to the water.
Jimlap has done a good job of laying out what is needed. One note:
...with a backless chair ....
Since many individuals with paralysis lack abdominal muscle control, a back on the chair would be required, but it could have a slot to accept the tank yet still offer back support. Since this is a public park you could make a cable and anchor one end in the water deep enough to have a 5 foot tall person chest to shoulder deep in the water the other end on land. Then have the cable play through a loop on a cable attached to the chair (Think dog run here.). That would prevent the chair from wondering off from mischievous children playing with it as it could only go back and forth into the water.The alternative is of course the lift chair off a dock type structure. A regular beach chair would work as the dive partner could bring the gear out to the water for an in-water gear up.
Ramps not more than 1:12 slope (1 foot of drop/rise for 12 feet of run)
ADA Accessibility Guidelines into the water. A water chair (usually made of PVC Pipe and weighted so it does not float) and of course is a toilet facility is available it should be handicapped accessible. Parking spaces for VANS with their 98" side space.
Not handicapped related, but In water you may want to lay cables or chains in a grid pattern like they did at Edmunsons in Washington State. This allows all to easily navigate the park. Add some structure items for blind divers to feel would be nice too.
The challenge here of course is getting the government entity that owns the area/park to allow you to do the project. ADA would require them to make it ADA accessible and will lay out the requirements.
Edmunds in Edmunds Washington
http://www.shorediving.com/Earth/USA_West/Washington/Edmunds_Park/ is a good model of how this can be done inexpensively. Cost to the local government was minimal as local volunteers have spent years building it as their weekend dive. it was a great dive when I was there. Chains and cables laid out in a grid with metal signs with the "street" name cut out of them placed where necessary so you knew where you were. Alternative, in Germany in the park they have things laid out in a orderly pattern of squares. At the corner of the square they have a monument with the numbers of the squares that meet there on it.
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