I installed my shower, and I connected the hot water supply line at the block drain plug hole. This is what the installation instructions recommend, and it sounds like your setup. I generally get plenty of hot water, but I think this is because of the shower pump and not any real pressure from the supply line.
Your mixture valve body may need some routine maintenance. If you have an Aquatik shower, each inlet (cold & hot) has a screen that keeps debris out of the valve body. When these screens get clogged, the pump can't pull water through them.
You might try disconnecting the three lines that are connected to the valve body and remove it from the boat. Then you can look into the inlet holes and see if the hot water screen is clogged.
I have to do this once or twice a year. I know it's time when there's not much hot water.
If the screens look okay, I'd make sure the block drain hole is clear and use compressed air to blow out the hot water supply line while it is disconnected.