We have taken DD's wheelchair onto those boats and have seen people with
ECVs on them. The old boats were smaller pontoon boats and could take wheelchairs, but not ECVs. They were replaced by larger boats that have specific parking spots for wheelchairs and ECVs.
Getting on the boats is usually faster because the CM puts out a small portable ramp and the wheelchair/
ECV is just parked on the boat. Busses take longer because the CM has to deploy the ramp or lift, get the wheelchair on and then strap it down securely.
When we have stayed at SSR, we just walked to DD. There is a good path and it lighted for night use. There is also a path from OKW, but I believe it is not lighted and not for use after dark. The path from OKW also ends up at the Westside area of DD, so if you are wanting to go to the Marketplace area, you will have a much longer walk.