Tango,I sometimes slip a piece of pipe over the drill bit leaving just enough bit showing to do the job. That way when the bit goes through you can't accidently drill into the stuff below (all things considered). A stack of nuts held in place with tape also works--my two cents worth.
Your comment about the sloshing water reminded me of a date I had as a kid. A/C was not common in the day and I had one of those nifty cylindrical swamp boxes hanging on the front passenger window. One quick manover in traffic turned my hot little date into little Miss frigid. Oh well, all was not lost as the episode turned into a wet "T" shirt contest! HA HA