sure, you can put a deck on it. maybe a hot tub and a couch and a even a tall entertainment stand with a 60" tv and a satellite dish on top. and as to how to make it more stable, after you do all your mods mount your boat on top of a 20' pontoon boat, that will make it stable.
advice is just an opinion based on experience. on this site there is plenty of experience, that is why it has been suggested not to put in raised decks. there is an old saying that applies here: "wisdom comes from experience, and experience comes from poor judgement." you speak of your plans as though no matter what advice you get you are still going to do what you want, so good luck, and definitely wear a life jacket, maybe two.