Don't do it.   Way too loud inside the boat.  On a car it works because nobody can see the motor, and they are left wondering - what's he got under the hood.   In a boat that only runs 50 you are gonna have people scratching their heads, and asking themselves "Why would anybody do that?"   But that is just MY opinion, it is worth what you paid for it.

Years ago--like about 20--I restored a '75 Rebel with a fairly stock 454 and a Berkeley JC. Before GPS, but I'd estimate top MPH to be in the low 60s. I also would not put in a gear drive, it'll still sound good.

With all different speeds with the acceleration and deceleration a gear drive is not preferred.

Just curious but why is that?

Ran a gear drive in my Ski-jet circle boat 30+ years ago. Sound was not an issue for a race boat and I wanted a solid valve train that was easy to tune.  For a 50 mph boat I would stick with a chain.  If you are aiming for being a poser with a "tuff" 50 mph boat put zoomies on it. But be ware anybody with more than a couple of years in jet boats will be using you for funny stories by the camp fire behind your back. Better to have a silent "stocker that goes 90.