I would think if it's only mud, crud, mucky floodwaters then maybe an auto shop strength solvent might be a bit much. I tend to take the "More Power" approach right off the bat

then again my solvent is Coleman white gas. I've tried MEK, methylethyl ketone before the grease and grime just jump off. But it is very nasty stuff.
One Question? do you know just what all they were submerged in? Can be a lotta nasty stuff in floodwater. Some of that crud/sludge could be eeewwww