An anchor pool was a form of a lottery. The organizer would take two sheets of paper that had grids of sixty boxes printed on them. On one sheet, he'd place a number in each box, from 00 to 59, as randomly as possible. He'd lay a sheet of carbon paper over that, and then fasten a sheet with blank boxes on top. One would often use a lot of staples or tape to ensure that nobody could wee the bottom sheet.
Then he'd sell the boxes, maybe a buck a box or, for high rollers, five or ten bucks a box.
The winner was determined by the time the ship dropped its anchor or passed over the first line or got underway. The winning number was the minute of the time that whatever event took place was entered in the Deck Log. The winner got half of the take. Those who had the numbers on either side got a quarter of the take.
They were called "anchor pools", but it could be any event that merited logging. Including the relief of a particularly hated commanding officer.
Anchor pools, of course, were gambling and were supposedly forbidden. But wise XOs turned a blind eye, so long as nobody was making a profit from them and the promotion of the individual anchor pool wasn't too blatant. Sometimes, official anchor pools were run to raise money for a charity such as Navy Relief. In that event, the charity got a piece of the action.