From Don Blair's book, Splashdown! NASA and the Navy,... 

  "Incidentally, when the crews of several of the Apollo 11 recovery helicopters invited me to join them at a popular Honolulu steak house prior to our sailing, they introduced me to the squadron drink, the Black Frog, and I still enjoy one occasionally 34 years later.  Two parts scotch and one part Kahlua... on the rocks.  Cheers!"

Ingredient

Meaning

Amount

Scotch

The smooth nature of scotch serves as a excellent base liquid

2 parts

Kahlua

Makes the beverage dark in color, like the wetsuit of a "Underwater Demolition Team" diver during recovery operations.   

1 part