The simplest way to extend runtime is to move up in mAh batterywise.
As others have mentioned, this requires cooling on the motors, because they will heat up substantually more, damaging the magnets in the longrun.

Better is to upgrade to a more efficient motor, the 540 motors are only 65-70% efficient (at best), the rest of your battery power is converted into heat.

Make sure the entire drive train runs as smooth as possible, steering outdrives are nototious powersuckers, a straight shaft or a flex shaft is much more efficient, keeping everything lubed is another thing, stuffing the stuffing tube with stiff grease is not a good idea, as it causes tremendous friction.

Choosing the right motor for the job is a variation on the theme above, a 800 motor in your Sintra would be very powerfull, but the boat would almost sink due to the weight of the 20 cells this motor requires.

Being content with less topspeed can extend your runtime considerably, my 12 cell monohull runs eight minutes with a 700 Neodym motor and reaches 45 Km/h
A test with a 700 ferrite motor gave me about twenty minutes of runtime with the hull getting just on the plane, not exitingly fast for a fast electric, but definately an alternative in a MTB that struggles with two 540 motors which get hot, even watercooled.

All components are related to each other and changing one will affect the others.

Regards, Jan.