I have a H280, and the pump is up by my water tank under the v-berth.
Does the water in the sink come on when you turn on a faucet? The "button" for the shower is just a diverter, directing the water from the faucet to the shower head.
If you are not getting water in the sink faucet, you will not get water out of the shower head.
It may be different on your boat, but I think many of them are set up this way.
Greg