The seat from the tower is about the best thing we have found. A friend of ours has used a walker in the past on their boat. worked good but takes up room.
Lots of snacks and water/pedialyte. We block the walk way to the bow so he can't get up there at all. He loves to help drive and honk the horn.
If you don't have to many people a pack and play works well.
We line the floor of the boat with soft coolers, he has already hit his head on the slider frame for pulling the seat back.
Eventually they fall asleep. Even then you need at least one person to watch at all times.
The life jacket its self is pretty restrictive. They have a hard time climbing up on the seats or siting up.