It might seem strange at first in 7 when racing underwater doesn't clear away Blooper's ink, until you realize that as an aquatic life-form, Blooper would have little use for water-soluble ink.
Some kart bodies give a speed boost in the air or underwater. Some are obvious, like the Soda Jet in the air or the Tiny Tug in the sea. It may seem odd that the Pipe Frame gives an underwater speed boost, but remember that water usually travels through pipes.
Mario Kart 7's gold tires and gold kart give a boost in weight when used due to how dense gold is in real life.
The Egg 1 in 7 offers a huge underwater speed boost due to eggs and osmosis.
It may be a bit unusual at first for an Arabian theme stage to deal with Shy Guys, but it has shout-outs to Super Mario Bros. 2, which was originally called Doki Doki Panic, which itself was an Arabian themed game. (And also the first true appearance of the Shy Guys)
Why is Waluigi Pinball in Mario Kart 7 despite Waluigi not being in the game? To make up for not being able to play on Waluigi Pinball online in Mario Kart DS!