"Marty S." wrote
I'm looking for information about electric motors and the most
efficient battery combinations in terms of weight of adding a
battery vs. the power that going to 24volt might give me.

I don't know anything about electric boats, but I know a thing or two about
electricity. The amount of useful work, ie boat moving, that you can get out
of a motor and battery combination is a function of voltage and current. If
you use more voltage, you need less current to do the same work. Conversely
you need more current to do the same work with lower voltage. A battery has
a limited amount of current in it that you can pump out into your motor to
do work. If you put multiple batteries in parallel, the voltage remains the
same but you have a greater quantity of current available to you. If you put
multiple batteries in series, you increase the voltage, but the current
capacity is the same as a single battery. Theoretically, it's a wash,
assuming that you're using adequate wiring so that you're not losing a lot
of power heating the wires. I suspect that the real differences we see in
run time depend primariliy on the efficiency of the propulsion unit, that is
how much of the energy that the battery squirts into the motor gets
converted to moving the boat and how much of it gets thrown overboard as
waste. I'm guessing that, all else being equal, typical 24 volt boat motors
probably have an efficiency edge over 12 volt ones because their internal
components are moving less current around and it's easier and less expensive
to manufacture an efficient low current device than an efficient high
current one. If you're using something to control the speed of the motor,
the speed controller's efficiency would also come into play and the same
rules apply.