As acceleration a = a0(1 - D/T), where a0 is the acceleration in a vacuum > a the acceleration in the fluid, D is drag, and T is the thrust (e.g., from water jet engines), we see that acceleration through the fluid ceases when D = T. We assume T is fixed so the limiting factor is in how fast V can the body go before reaching D = T, which is when the terminal velocity is reached. Clearly, when Rho > rho, D = T will be reached at slower velocity than when the body is moving in air with rho as the density.

And there you are. The terminal velocity in water will be lower than the terminal velocity in air. Yet the speed of sound in water is higher than in air. Bottom line, until we can create thrusts about 800 times that of our aircraft engines, an underwater body will not even reach comparable speeds of aircraft, let alone bust the sound velocity which is even higher in water.
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It is just MUCH more difficult to do so in water, because water’s speed of sound is much greater, and water fluid drag effects will hinder you a lot more.

It is possible to go that fast in water if you "try hard enough". Throw plenty of structural design at your vessel, throw plenty of energy at your propulsion, and you could break the hydrosound barrier. It is just impractical to do so.
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