You can also get a venturi vac attatchment for your pressure washer, just run the pwasher water through the kit and drop the suction part of it into the dirty water fresh water you pump via washer comes out of end mixed with the dirty sucked up water. Be carefull where you discharge it though!
Regarding the drain plug, a diesel tank should slope slightly one way. The lowest drian off should be at the lowest level, this is used to drain the water which is heaver than fuel so settles to the bottom. The fuel should be drawn from almost the bottom of the RAISED end so you never in theory pull water out, it just lies harmlessly in the low end and can't be got at unless you remove the lowest plug. Unless of course years go by or water gets in enough to be higher tan the fuel draw off