Why would you want a discharge valve? The chances of water pushing back up the line when the discharge thru-hull is below the waterline (while sailing) is not good, there is nowhere for it to go.and you don,t have to read a gage to SEE the water coming out !!!! ;-)
If you forget to open the discharge through-hull valve, you can/will get ~800 psi or more 'just before' that discharge exit. Such are the 'features' of 'constant displacement' pumps. :-o
The outlet brine is not at 800PSI because it is only the residual water that has squeezed past the pressure control valve. My discharge outlet is 100mm above the water line and the water that comes out is not unlike the average garden hose. I don't have a discharge valve, just a skin fitting. I very rarely run the water maker when sailing, only when motoring or anchored.