Gurmag Drowner

You can sacrifice the creature with exploit if it’s still on the battlefield. This will cause its other ability to trigger.

A creature with exploit “exploits a creature” when the controller of the exploit ability sacrifices a creature as that ability resolves.

If the creature with exploit isn’t on the battlefield as the exploit ability resolves, you won’t get any bonus from the creature with exploit, even if you sacrifice a creature. Because the creature with exploit isn’t on the battlefield, its other triggered ability won’t trigger.