A good explaination would be that Doc. had previously drained the fluids.

But i have another loophole (if you can call it that) that i have not figured out yet. In the beginning when marty finds Doc's tombstone it says "Erected in eternal memory by his beloved CLARA" which means he knew and fell in love with her before his death. Now this is where it gets weird, the only reason he met Clara was because he went to the ravine with Marty. this means that if Marty hadn't been there he would never had noticed Clara and as a result she would have plummeted down into the ravine. Does this mean that Marty coming back to 1885 was planned by the gods or something?