Sharking is a trick in GoldSRC, as well as New Engine, which allows the player to bounce on top of the water with minimal speed loss. It is not possible in Old Engine, as the player will simply lose all of their speed when attempting to shark.
Sharking is one of the simplest tricks in Source and GoldSRC Speedrunning.
Right before landing in water, the player simply needs to start holding their jump key, which will cause them to bounce on it.
An example of sharking being performed in Half-Life 2 is available:
https://www.youtube.com/watch?v=fdOrRJ0BuC4