An underwater dash allows you to jump underwater with same speed as being in air. To understand this, you need to know that the first frame you jump underwater, the water has yet to take effect on you. Or, in other words, you are still at air speed on the first frame. In addition, whenever you are holding L and R simultaneously, your speed and direction is always locked.
So, combining this knowledge, if you jump while pressing L and R the first frame during the jump, you can lock your jump at the speed it would be at in air. This is how to perform an underwater dash. Also keep in mind that you need to continue holding L and R during the jump to maintain speed. In general, the easiest way to do underwater dashes is to walk in a direction while already holding the L button, then just trying to press B and R on the same frame. The most significant use of this technique is performing Early Gravity Boost in Hydrodynamo Station.
You can achieve an odd situation where it looks like water has disappeared with a technique called air underwater. Doing this technique creates an odd environment where your uncharged beam shots immediately hit an invisible wall and the sound is clearer as if you weren't in water. To perform air underwater, you must morph on the frame the camera enters the water, making it a somewhat difficult one-frame trick. This can be done while falling into water or while walking sideways into a new water box. It is possible to do this trick with Gravity Boost, but it is much harder as there is no "buffer" for when to morph.
The most important aspect of this trick for speedrunning is that you can Screw Attack underwater, which usually is impossible since Gravity Boost normally replaces Screw Attack while underwater. So, this technique provides another (faster) method of doing Early Gravity Boost in Hydrodynamo Station. Moreover, it is optimal for the Alpha Blogg fight after obtaining Gravity Boost, since you can perform air underwater before the fight and one-hit kill Alpha Blogg with a Screw Attack. As a result, you can access the Power Bomb Guardian from the closer of the two applicable portals, saving approximately 10 seconds compared to the other portal.
Reverse Air Underwater
An interesting variation of air underwater is called reverse air underwater. This trick is similar in concept to air underwater, except that you have to morph on the same frame the camera exits the water. This makes the air above the water all murky. The big use for this trick is being able to climb to unreachable locations given certain item restrictions.
Gravity Boost is required to pull off this trick. One reverse air underwater is achieved, fall onto any surface of water and press and hold B before falling in. If done correctly, Samus will activate Gravity Boost and fly through the air. Unfortunately, this trick is quite limited, as the distance you can go is determined by the duration of the Gravity Boost jump. Its current relevance is in randomizers for getting some items under certain restrictions, such as Path of Roots Missile without Screw Attack or Grapple Beam.