Alternatively, throw a bucket of water over your model...
Seriously, though: I can't think of a step-by-step way to do this in Photoshop because (as agentxi has said) the physical characteristics of wet hair are so different from those of dry hair. You might be able to achieve something with the Liquify tool, but it would be such a time-consuming and intricate process that you'd probably go mad or blind (or both) first.