Several issues come to mind.
1. What do you mean by low pressure? Is this at a low flow (perhaps using a hose nozzle set for low flow) or is it out of an open hose? With a variable nozzle you should find a setting that gives you an adequate stream velocity. If not, perhaps you do need a dedicated washdown pump.
2. If your water pump is well aft, then you will have a considerable length of line from the pump to the deck wash. This will contribute significantly to low pressure.
3. What is the rated delivery
(gpm) of the pump? If it is low (a few gpm) it will not likely develop much pressure at the higher flows usually demanded of a deck wash. Your galley
faucets are possibly designed for lower flowrates and thus have higher output pressures.
4. There is certainly a possibility of a kink or other blockage in the lines leading to the deck wash outlet.