I am somewhat familiar with such issues and my first question is if the intent is to help a person with a disability to access a boat or is the issue to made a facility ADA compliant. The second task is much more complex. The key here is accessible versus legally compliant.
If you question is about the first point, the simplest approach for different vessels is to install a lift chair ( looks like a davit with a seat) on the dock assuming it is a floating dock or on a fixed dock assuming the tidal swings are manageable). The kind of chair I am mentioning is the type you may have seen at public pools or on cruise ships. Those chairs start a couple of thousand dollars plus installation. The dock may need work to be structurally adequate for the loads.
Here is a link to one product as an example. I am not endorsing or recommending it. http://aquaticaccess.com/?ibp-adgroup=a ... oCcifw_wcB
Of course, some new vessels have gunnel doors or in the case of pontoons, railing doors. With proper portable ramps, some boats would be a lot easier to board and disembark.
Attaching to the boat is an interesting idea but poses some challenges. Structural strength is one but I suppose that can be addressed especially if additional support is provided by some sort of gunnel mount in addition to a floor mount. Some of the manual units have the ability to be removed form a mounting sleeve similar to deck tables. I think your idea has merit but would need input from the various manufacturers.
Keep us up to date. This is an interesting topic to me.