Hydrodynamic is the right word. The notions of hydrodynamics and aerodynamics parallel each other: one is to air or gases as the other is to water or liquids. Here are the Merriam-Webster definitions:
: a branch of dynamics that deals with the motion of air and other gaseous fluids and with the forces acting on bodies in motion relative to such fluids
: a branch of physics that deals with the motion of fluids and the forces acting on solid bodies immersed in fluids and in motion relative to them
And hydrodynamic has been used to mean the qualities that allow easy and fast motion through water. Here’s an example from the Daily Beast (my emphasis in all quotes):
These suits also made the body shape very smooth and hydrodynamic. Instead of the joint between a man’s body and the waist cord of his swimsuit adding extra drag, there was now a seamless, wrinkle-free, low-resistance outer shell skimming through the water.
Wordnick has a few more examples of usage, some of which are in the sense you want, such as (from the ImpactLab.net):
Like its natural archetype, the AquaPenguin from Festo has a hydrodynamic body contour.
Now, aerodynamic appears in some dictionaries with the relevant sense. See Oxford Learner’s Dictionary or The American Heritage® Science Dictionary (2002) via Dictionary.com:
Designed to reduce or minimize the drag caused by air as an object moves though it or by wind that strikes and flows around an object.
Merriam-Webster has the corresponding noun. On the other hand hydrodynamic does not appear with the precise parallel sense in any dictionary I have access to. I could find only the more general meaning. See several dictionaries in Dictionary.com or Merriam- Webster:
: of, relating to, or involving principles of hydrodynamics
The same is true, however, of aerodynamic in some dictionaries (see Dictionary.com), which give the general definition of aerodynamics — branch of dynamics dealing with gases — and simply list the adjective without defining it explicitly.
As to whether it is wrong to use aerodynamic for an underwater object, aerodynamic and aerodynamics are defined with respect to air or gases in every dictionary I could find, so I don’t think it appropriate to say that, say, a submarine is aerodynamic. Suppose the submarine is hydrodynamic and can be sustained in the air by some external force that does not interfere with its motion. Let’s assume for the sake of argument than the features that allow the submarine to move with minimum drag in water would also allow it to do so in the air — whether this is true or the comparison even makes sense I don’t know; physicists and engineers may want to weigh in on this. Then the statement the submarine is aerodynamic is true; but to my mind it only says that the submarine can move with minimum drag through the air, even though a well-informed person could tell from it that it would also move with minimum drag in water. That’s my two cents anyway; philosophers may want to weigh in on this.