I too have known keyboards to go in dishwashers ok. But a keyboard isn't the same as a motherboard. Generally there is much more to lose with a motherboard.
Its not the water that kills things, its either what is dissolved in the water, or what is on the board that dissolves into water that is the problem. If you can get it completely dried before putting power back to there is a pretty good chance everything will be fine. But, using anything except really high purity DIW (and even with that if its not air cleaned first there is going to be a lot that will dissolve and make it no longer DIW ) is going to have the chance of having something conductive dry out right between 2 areas that aren't supposed to have any electrical contact. If that drying part is just calcium, then its probably fine, but if its anything conductive it can be a problem.
Maybe the chances are low, but is it worth the risk?
What has been done to this motherboard to necessitate cleaning by anything other than air or maybe a swipe with a lint free cloth?