Hi @Llammers, welcome to the forum 
We generally don’t recommend running T200s above their rated 20V, but if you’re able to keep the throttle low it should be possible. Alternatively you could regulate the voltage to 20V or lower, in which case you could use the thrusters at full throttle without the same worries about the thrusters or ESCs overheating and becoming damaged or having shortened lifespans.
That said, the main concern with batteries is generally the current capacity. T200s can draw a significant amount of current (especially at higher voltages), and it’s important to ensure your power supply has sufficient burst capacity to handle the maximums, and sufficient continuous discharge capacity to handle the levels you’re expecting for prolonged use. I’d recommend checking our T200 Technical Details to understand the performance and power draw at different voltage levels and input commands. Consider also the weight and shape of your boat, and how that determines the speed/control you could get from some installed thrusters.
This thread discusses lead acid batteries for powering T200s, so may be relevant/of interest