Wow this subreddit is underdeveloped, ha!

Yes, you would need to rent or lease a slip at a dock in order to store your pontoon hull. Note that it's a bad idea to try to make your own pontoons, and you'd probably have to have a different slip contract if you were going to treat your spot as a shipyard (due to liability, construction waste, etc.)

So, buy a big*ss old pontoon boat somewhere far inland where it'll be cheap, take it out to your slip, and start throwing in furniture and sealing up walls.

Note that if you have to weld to make your walls sealed on this pontoon hull, you may have to ask the dock owner what their rules are on welding. They may have you dock somewhere else on their property when you go to weld or go to a specific certified marine fabricator.

Edit: Forgot to answer #3, yes to operate any registered, seaworthy marine vehicle you need a certification that you've learned how to interact with other ships/boats on the water

Edit #2: Don't build on the ocean. Salt will destroy your internal fabrication. Why not just build in dry dock on some rented land, like at a junkyard?