Prevent corrosion of circuit board fallen in water
You need to wash it.

Seriously, what will cause corrosion is any salts that were dissolved in the dirty water that have remained behind when you air dried it.

You need to buy some distilled water and repeatedly wash the board a few times in fresh distilled water to dissolve and remove as many of those salts as possible. Flushing under components may require the use of something like an ear syringe, or as Peufeu suggests, a dental water jet.

Then, once the thing has had sufficient time to dry, and if there is a risk of this happening again, you should coat the board in a urethane, or better.. epoxy, so this will not happen again. 

TIP: It is an old trick to stick your board inside a bag with a desiccant for a few days to completely remove any moisture trapped inside hidden places on your board, wires and other components. Rice apparently, despite the rumours, is not a great choice and there are better desiccants just don't use a salt, and keep the desiccant itself from directly touching the parts.

NOTE: Salts + humidity = electrolyte. Not only will that corrode stuff, but it also conducts which can cause shorts, arcing, and function/part failure.