Your boat if it's a Grady can take all the saltwater blasting you want - it is only used for cleaning the deck, gunnels, hands ir blood, and fish cutting activities on transom. No reason to be using it for anything else or anywhere else. If you want to wash out the bilge spaces, use the dock FW hose. For swimmers on Gradys, I bet they would prefer FW washdown.
You can add a 3 way valve and convert the existing pump whatever it is to washdown. I presume it has a pressure switch. Even if it's designed for LW it will still throw a decent spray. When going the other way it can be a problem since washdowns are not designed for continuous duty as a LW, unless it is so designated on package. As mine is.
Wilcox makes the valves found on older Gradys, I used a Lowes wide bore plastic garden 3 way valve and added the fittings for the 3/4 marine hose, at a fraction of the cost. Been on boat about 3 yrs now not a leak. The you switch valve to use LW. If you think you will get pressure at both LW and washdown, you will get zero flow to LW when hose nozzle is activated for washdown, and visa versa.
Alternately, you can add another pump and new 3/4 inch thru hull fitting and valve just for washdown, and new elec feed. A pump designated for washdown only can typically be found at higher pressure ratings.