[A] half-living tank, forged by the Kuo Toa (or any other evil sea-dwelling race, should you wish) by surgical and magical means. A live giant crab is taken and a chunk of its insides are taken out to create a small transportation area where its handlers can sit.
The life of the mind flayer begins and ends in the tank of the Elder Brain. As little tadpoles, they’re placed in its tank, where it feeds off their psychic energy. Those who survive get to become fully formed mind flayers. At the end of an mind flayer’s life, the brain is removed from the creature’s head and placed in the tank, where it’s absorbed by their leader.
I believe both of these monsters were introduced in 3E and are official D&D monsters (though they were new to me). They are both great ideas. I have still never run a heavily aquatic scenario, and it’s something I would like to try at some point. I can imagine rather than tanks, what if the elder brains inhabited great airless flooded caverns and tunnels? This would necessitate descending into the watery depths to actually defeat an illithid colony; sort of like a fantasy Lovecraftian version of The Abyss.