Shrimpy Truffle

This comes out of Duke Fishron on Expert and above, and what it summons behaves like two separate items depending on one condition. Dry, Cute Fishron flies forever but accelerates lazily and turns worse than most wings. Wet, it is the fastest way to move in the game and it hands you a damage bonus while it lasts. Everything about using this mount is about keeping the second version switched on, which is why it is excellent in an arena you built and awkward everywhere else.

Any liquid triggers the boost, including honey and lava, and it hangs around for a few seconds after you leave. Rain counts too, so a storm is a good reason to go start a fight you were putting off. Otherwise you build for it: a shallow pool to dip through mid fight, or bubbles filled with honey scattered around the arena so you pick up healing alongside the speed.

Pouring a bucket over yourself in midair works, with the catch that the water has to land on your character rather than on the mount's head or tail. Expect small puddles to settle wherever you do it. In multiplayer someone with a Water Gun can keep you soaked for free.

Dropping below half health switches on nearly the same speed and the same damage bonus without any water involved. That sounds like a trap and normally is, but a Frozen Turtle Shell or a Frozen Shield exists precisely to make sitting at low health survivable, and a Rod of Discord will put you there on demand. Built deliberately, it means the mount is at full power for the back half of every fight that is going badly, which is when you want it.

Touching any block drops it to a walking pace until you get clear again, boosted or not. In an open sky arena you will never notice. In a cave, a corridor, or anywhere you have to thread past terrain, it is unusable. The one thing that crawl is good for is laying a line of blocks in front of you at a steady pace. On platforms it will crawl if you start there, though it cannot land on them from the air, since it phases through the way other infinite flight mounts do.

The UFO is marginally quicker in dry air and it never has to think about water, so for general travel and exploration it is the safer pick. This one wins the moment liquid is involved and wins by a wide margin. If you want mobility that does not depend on a condition you have to maintain, wings or the Witch's Broom are less trouble. Sharp turns are painful while unboosted, and the workaround is to dismount in midair, dash the other way, and put the mount back on.

You can tell the boost is live because its eyes light up, and the glow fades as the timer runs out, so it doubles as your only readout. The color follows whatever the water looks like in the biome you are standing in, and a Water Fountain will change it. During a Blood Moon it glows red no matter where you are.