This isn't a fix for the water leak, but might make your life a bit easier. Why not replace the manual valve with a high pressure/high flow asco solenoid valve that you wire up to a switch on your dash. Call it a master shutoff.
Previous: 1993 Prostar 205
Red 1998 Ski Nautique, PCM GT40, 310 hp, , Acme 4 blade, Perfect Pass SG/Zbox.
Be kind. Have fun.