4 screws on the front and the stove slides out forward, you can slide it out enough to gain access to remove the gas line and completely life it away.
I'm not entirely sure if it's actually enough room to replace the hot water tank though, I believe the tanks are installed before the top side is put onto the boat. If it's anything like the removal of the hotter tank on a friends C30 he needs to cut apart the existing tank and replace it with a smaller replacement tank.