First, turn off the device immediately. There's a chance nothing is wrong yet, but if water seeps in while the tablet is still running, it might short and fry a circuit. So, turn off the power and make sure there's no electricity flowing in those circuits.
If you need to use your tablet "asap," you might be out of luck. The safest way to ensure your Playbook keeps working might take some time.
I recommend you find some uncooked rice or silica packets (the little packets labeled "Do not eat" that you get in things like new backpacks) and encase your tablet in that. This will serve to absorb the water. Leave it like this for some time, preferably an afternoon or a couple hours at least.
After a while, take the device out of the rice/silica and disassemble it. You're going to go hunting for leftover water and anything left behind when the water evaporated. Check the circuit boards, battery, and electronics for traces of water and clean them off with a cotton swab or microfilter cloth and isopropyl alcohol.
Once you've thoroughly checked the device and ensured it is dry and clean, try turning it on. If things don't work from there, try the [[Blackberry Playbook Troubleshooting|Troubleshooting Guide]]. At that point, though, it's pretty likely you've fried the battery or, worse, a circuit.