Ok.
Your running 6volt.
Throwing numbers around for illustration .
If your motors require 200watt for efficient operation ,at 6v the motors would require 33 amps
If your motors require 200watts for efficient operation ,at 12v the motors would require 16amps
voltage can change ,but wattage remains the same ,more voltage less current, less voltage more current.
I would suggest ,you try running a 12volt system ,or what ever your ESC's can handle , and give it a go.
Heat is a good indication of excessive current use.
You can root around the internet ,its liklely ,very likely there is a 12volt battery with the same foot print as you 6v battery.
It wouldn't be that hard to just trial the boat with a 12v battery ,borrowing a battery from a friend, just for a look see.
from where im standing , excessive current draw will toast your battery ,as it will only discharge at a certain rate.,It will also have a larger voltage drop, less voltage ---more current ---because of the wattage.
or
Switch to 6volt motors,however 12volt operation ,these motors can stand.
http://www.mabuchi-motor.co.jp/cgi-bin/catalog/e_catalog.cgi?CAT_ID=rs_540sh