What is the model of the boat? That will determine where the seacock is located. In a 2700,2800, 2900 they should be in the mid-bilge just outside of the head. The seacock would need to be open an the boat in the water in order to draw raw water in. Unless it's a vacu-flush.
There might be a red, lit switch in the head for the shower sump as well. This would not be related to the pressure water to the faucets.