Removing data from Macbook pro

First off, hold the power button down 30 seconds without any power attached.

Then plug in the power and tap the power. If not so much as a whisper comes from it, take it to Apple. If it tries to start up, immediately hold

command-option-shift-R and don't let go until you get feedback on the screen.

I would never attempt to trade a water damaged machine. A: the storage on the computer is directly on the logicboard. B: if it won't boot as a result of the water damage, there is little likelihood the machine has much value, since the entire logicboard will have to be replaced. You are better off trying to make a claim if your homeowner's insurance will cover it, or if you have AppleCare+ protection with an accident protection. You could try internet recovery to see if it will at minimum let you erase it with internet recovery's Disk Utility. command-option-shift-R. Short of that, only a service center can get direct access to the drive to erase it.