I have heard the watermelon sound! I gut shot a deer late one afternoon and heard that same sound, went back the next morning and found the deer approimately 150 yards from where I shot it. He had cross a creek and just made it up to the top of the bank when he died. I think the cold water rushing into the wound helped put the deer down due to shock. If you run out of blood try looking around for the closest water source.
If the deer has not been pushed it should have bedded up pretty quickly.

Good luck!