As understood by the Federation's Terraform Command, an hydraulics specialist was a member of a terraforming project that dealt with establishing a source of potable water.

In the case of the operations on Velara III, one such specialist was known to be Arthur Malencon. In the course of his duties he had to tap into the planet's water table by using the master subsurface pump and the hydraulic probes. (TNG: "Home Soil")