When you say "battery powered", I'm assuming you mean internal batteries rather than the boat's battery. If this is the case, there are lots of units on the market, but they only have a short battery life (typically less than 20 hours). This is because gas detectors are quite power-hungry, due to their mode of operation.
Thank you very much. It seems that I'd better get an alarm that's connected directly to the boat batteries. The gas system are equipped with a double on/off valve and a leak tester.