Gravity Boost

From Wikitroid, the Metroid wiki

The Gravity Boost is a power-up for Samus in Metroid Prime 2: Echoes. It helps Samus move through the water with ease, and letting her do a triple Space Jump underwater (it actually makes her float, but it does the same job.) It was located in Torvus Bog, at the Hydrochamber Storage Room. When Samus collects it though she will have to fight the Alpha Blogg, which is the guardian of the item.

The Gravity Boost, (which has a hint in the name) has a similar effect as the Gravity Suit in the games that feature it, except the third jump that is possible underwater with it that is exclusive to this game.

It should be noted that in one cutscene, after Quadraxis' head is destroyed, she used the Gravity Boost to jump away from the exploding head. This is totally contradictive to the fact that it's exclusively for underwater use.