Hull through

On the boat I have bought, the toilet hull through is just a pipe (visible from outside) welded to hull and inside there is no seacock. My boat has shallow draft so its easy to get above the submerged line, so I am thinking of cutting this pipe out and welding a bigger 6/4" pipe in there to be sure its healthy and place the ball valve above line of submersion, so its not in actuall direct contact with water and can be changed when on water. There is a similar thing on the kitchen sink, where the pipe goes above the waterline and the ball valve is dry. I think there is nothing to crack this way. Do you have ideas about that?