Because someone may find this interesting, the main set for my alma mater was drop down shooters yesterday. Because the terminology used varies, shooters for us were 25s underwater w/ fins.
First one was on :50, second was on :49, third was :48, and continuing to drop the sendoff 1 second until you failed. Sounds like a bunch of guys were right around :20 when they stopped, and the "winners" made it down to :17.
I'm kind of jealous, a few of us really campaigned for this set the last 2 years before I graduated and we never did it.