Job 13:
S.T.E.A.M.
Submerged
Technological
Expertise
Analyzing
Magma
A subsidiary of Gadd Industries, this course takes place in an underwater research facility near an active volcano. The drivers race through brightly lit hallways that lead to various sections like laboratories and water treatment areas (kind of like the Wario Dam section of Mount Wario. The interior, not the exterior). The drivers are eventually led outside of the facility into an underwater section where they drive around the exterior and up an erupting underwater volcano, which serves as a boost that leads them back to the facility.
This course contains rare, special item boxes containing prototype FLUDD nozzles that automatically attach to your kart when obtained.
Blue boxes give you the Squirt Nozzle, which briefly attaches to the front of your kart and sprays a stream of water that spins out opponents if they're sprayed for long enough. You can control the direction of the spray with the right stick.
Red Boxes contain the Rocket Nozzle, which boosts you upwards. These are usually found near branching paths, where obtaining one automatically boosts you up to an alternate route on an upper floor.
Gray Boxes give you the Turbo Nozzle, which briefly attaches to the back of your kart and sprays out water backwards to increase your speed. You can even pass people and drive in front of them to spray them with the water and spin them out!
Abstain from picking a retro course. No ideas here.
EDIT: Added nozzle mechanic