A waterlogged phone will likely never work again. Your only hope is to put the phone in a desiccant, such as rice right away and leave it there for a week. Even then this is a low yield save mechanism.
If you want to deactivate the phone from Find My iPhone you can do this from icloud.com
Agreed with Weth. You can pull the sim card and that will ensure no on could use the device if it turned back on. I'd suggest deactivating it via iCloud.com as mentioned above especially if you do sell it (someone would purcahse and repair it) or trade it in or recyclce it via Apple.