Interesting. How long is the boat? Totally agree safely dispose of the batteries. What is the black tube around the prop? Previous owner trying to create a jet drive the hard way? Not sure what all that stuff is on the back of the boat but I would get rid of all of it. Looks like it has potencial. I would also post this over on
https://forums.offshoreelectrics.com/forum.php . Lots of knowledge over there. If you post over there give as much detail as you know about the boat and they will steer you in the right direction.
I'm guessing that boat is almost 36" long. Should run good on 3 to 6's. With probably a 120 to 180amp ESC Motor at a guess maybe 2000 to 3000 kv. Unfortunately I'm just guessing. I'm still new myself to boats . The boats I have , have 2 motors each (Catamarans).You may need a lower KV motor because you only have the one motor.