Hi Den,
I use these Nickel Metal Hydride buggy packs for most of my scale boats, they are 7.2 volts and come in various capacities the more you pay the more running time you'll get , and they can be fast charged and weigh 13ozs.
The little Sealed Lead Acid battery is 6 volt and 1.3 amp hr is slightly lighter at 11ozs and has to be trickle charged.
Howes of Oxford have a page in Model Boats advertising all sorts & they have a Website.
Have you finished the Eventide yet ?
Regards Ray