Parallax wrote:

I remember in "Run Silent, Run Deep", the sub motor could run with
batteries in either serial or parallel, not sure why. Serial would
give lots of Volts perhaps for lots of power whereas parallel would be
for extended cruise at low power. Could such a system be used here?

Why? The speed of these electric motors is dependant on the voltage (RPMperV). Serial =high speed, parallel =1/2
speed at twice battery capacity.
I can't see why such a system could not be used on an electric boat.