To swim very fast, you must be moving really fast before hitting the water, and cannot bump into any objects in your trajectory once under water. The best way to do this is to fall from a really high height. Go to the shore, perform a Melee Dash with the Jetpack Jump and keep going up until your jetpack runs out, then keep holding forward once you hit the water. You can then turn and pitch your camera into any direction to maintain your falling velocity underwater. If you are in an underground cave and cannot get a lot of height, use sprint and Melee Dash into a bunnyhop to build up a lot of velocity before you hit the water, then point your camera into the trajectory you want to go once you are underwater.