Use DI water, and rinse extremely thoroughly if you have to use soap. You want zero residue, and as such I don't recommend soap at all.
DI water = deionized, or distilled. Tapwater will leave lime scale and/or chlorinated substances that can and will eat your PCB over time. Plus, if there's internal capacitors or batteries that hold a charge, there's a low enough resistance in tap water to discharge those, possibly to the point where you burn out paths in some of the more sensitive chips.