my parents both have mobility issues and I would never go on a WDW trip with them... it would be miserable (not just for the mobility issues, my dad does not travel well either so add both of their mobility issues and the heat and crowds and bleh.)
But I did want to take my son on a vacation with his grandparents, so I chose a cruise for a low stress, low toil vacation. It was amazing though we did get a wheelchair for my mom because at the time she could only walk about 10 feet on her own.
I live on an island and spend more time on boats than average and the Disney ship is not like the "boat" your dad is thinking of. There are no ships ladders, no hatches with trip hazards (though the thresholds in staterooms are a bit high but not like hatches), no bunks to climb into (unless you want to sleep in the one bunk in a room), no pump toilets where you require strength and leverage to flush it. Being on a smaller boat (even one meant for overnight and longer) can be utterly exhausting but being on a cruise ship is entirely different. It's like a giant floating luxury resort and WAY less taxing than a WDW vacation.
I spent 5 nights on a schooner recently and I have never been more exhausted in my life - it took me a full week to recover from the experience. It was nothing like a cruise.
I would do either a 4 day bahamian or 7 day eastern caribbean cruise as both include a stop at castaway cay, the private island where there is even a tram to take you places. If he did WDW without an ECV he won't need one on the ship, there is much less walking. if he's wobbly for the uneven sand, there is a sand wheelchair to take him out to the sand. I would not miss castaway cay on a first
disney cruise - it's as safe and accessible as you can get for a beach vacation.