My original point was that 400 mph through 'water' seemed too high...
Think so?
Don't know... perhaps.
There are too many unknowns with the antenna. The terminal velocity in water would be dependent on several factors:
1: initial velocity of the antenna on impacting the water.
2: Mass of the antenna.
3: Presenting cross section of the antenna related to orientation of fall.
4: The viscosity of the seawater.
5: Volume of the antenna (volume and mass may result in a specific density that is less than that of seawater giving the object bouyancy.)
6: Temperature of the various layers of seawater and any rising or falling currents.
It is possible for an object that is sufficiently heavy AND sufficiently thin (streamlined) to reach a very high velocity in almost any fluid (for this purpose air and water are both fluids)... but we need to know this and other information to calculate a terminal velocity.