So, Blue Submarine No.6. Other than the nightmare-inducing CGI, it's also notable for being furry/scalie bait, complete with the fish lady, Mutio.
Most of this bait is thanks to Dr.Zorndyke, a brilliant genetic engineer, who decides that it's time for humanity to be replaced and orchestrates a polar shift to flood the Earth's coasts, killing billions, and we can't even sell their now underwater apartments.
So, Zorndyke also has an army of hybrid creatures, including the Musucas (see pic related for more info). Musucas serve as transport for the Water Spiders, nimble mechanized troops that, along with the Ghost Ship, seem to be the primary offensive force. We know little about Musucas, but they were grown in underwater "cultivation tanks", near one of the poles. They are sentient and sapient and capable of verbal communication and probably have organic sonars as well.
This is what happens when you max-out Google-fu. Here's a video of one, though it has spoilers, so...
But, would that be workable in a realistic setting? Musucas aren't designed to engage in direct combat and when they do, the mortality rate is 100%, though they can score kills. Their role is transportation, the question is what (if any) advantages would such bioships have over modern, conventional waterborne military vehicles (larger ships and submarines)?