This map requires Snack Pack 3 to play.
Download here: https://www.slipseer.com/index.php?resources/snack-pack-3-trick-or-treat.280/

Instructions: 
Just place the contents of this zip into snack3, and launch the map via console with "map lns_spoot" without quotes.

There are differences in skill difficulty, so make sure you've chosen the difficulty you want to play with before launching.

P.S.: it is recommended that you bind separate keys for "swim up" and "swim down", as there will be some combat under water, and it helps to be able to swim up and down properly, without needing to look away from the monsters. 