What Battery ?
Welcome to the site.
Batteries come in three major types, Sealed Lead Acid (SLA), NiMh and LiPo.
SLA's are the heaviest and LiPos the lightest. All are available in a variety of shapes, voltages and capacities (current). As you have an older model it is possible the speed control may be limited to a maximum voltage so to give useful help we need to know details, please?
6v SLAs are readily available as are 7.2v NiMhs. if the motor is a similar age to the boat, chances are it will require lots of current, but if you can supply more info or a picture we should be able to guess its identity. A picture of the inside would also help identify how much space is available.
Dave