I didn't give it much thought when I said tap off 2x6volt in parallel. I will probably use small 4s lipo to power lights etc.
Thanks fo pointing this out. Still a long way from installing ancillaries.

Alternatively, pick a sensible master voltage for the boat, and use step-up/down converters to provide auxiliary voltages for other purposes? They provide efficiency in the 90% range, and power of about 5A for a couple of quid. I have been doing this to get radar dishes to rotate at precisely the speeds I want....

You can power a brushless from lead acid batteries electric is electric. Lipo batteries are often used because of their ability to provide power with a relatively low weight and can dump LOTS of amps.
As Colin pointed out 2 6volt batteries in parallel will provide 6 volts and cannot be tapped for 12v to obtain 12v the batteries need to be in series and 6 volts can be tapped off but will only provide the energy from one of the two batteries.

Hi Dick, two 6v batteries in series wil give 12v, which you can tap off 6v if required. But not in parallel I'm afraid. Hope this helps you.
Cheers Colin.

Hi Havelock, this is my first boat build,been doing rc model aircraft for 60 years,I know all there is about brushless motors and lipos.The kit specified lead acid batteries and I am using a 2_1 mfa motor and gearbox not quite sure what esc I have got. Lot of this was guesswork.The lead acid batteries will complement whatever ballast I need.2x6v in parallel and I can tap of 12 v for lights smoke etc I think.

if your intending to use a car ESC have a good read before you buy some of them require you to select reverse then go back to neutral before selecting reverse again to actually reverse the motor. The first reverse acts as a brake. Just to satisfy my curiosity why did you want brushed rather than brushless? There are a lot of brushless motors used in the model car sport and generally have advantages over brushed motors.

Latest update on tug.

Thanks for your replies I've purchased mfa 2-1 500/550 motor.has pulley drive to drive shaft pulley

Hi Dick, I have a few tugs and the largest at 49" uses a geared 540 running from a 12v 7ah sla battery giving a good 90mins running time. This model weighs in at 15 kilos ready to sail. I also have a caldercraft model tug Joffre. Which is about 30" long and runs using a 6v monoperm motor and gearbox. 6v 4.5ah sla battery giving about 1 hour run time. Weight is 5kilos ready to sail. Hope this helps.
Cheers Colin.

Dick, as far as I know model boat/car style ESCs have a reverse function.
I would think a geared 540 motor would be quite sufficient. People tend to overpower most model boats, especially those not required to take off like ducks on acid! Like tugs.

Cheers,
Martin