20an early two in one
It keeps you on the surface of water instead of dragging you under, and it also hands you the Flipper effect whenever you are in liquid. Early on that covers what the Flipper and Water Walking Boots would otherwise cost you two slots to get. Hold down to sink on purpose when you actually want to be under.
when to drop it
There is no upgrade path. Everything that replaces it goes through the Tinkerer's Workshop, so once you can build proper mobility accessories the tube moves to a vanity slot or the trash. A handful of effects also override it outright, the Merfolk transformation and Fishron Wings among them, so it can quietly stop working without you noticing.
the launch trick
Underwater it pushes you upward with no speed cap at all. Dive to the bottom of the Ocean, let go, and you come out of the water faster than anything else in the game will move you, high enough with a Featherfall Potion to reach a Floating Island. Past a certain point you start clipping through blocks. It is also the basis of the lance damage exploit, since lance hits scale off how fast you are traveling.