I thought at first there might be some similarity to an "endless pool
". Now that I see the picture, I agree - but even a large bathtub is better than none!
As a matter of fact I just tried such an endless pool at the hotel where I spent the Easter holidays. The jet turbine at one end creates a current against which you swim as long as you can
. Now, I have no doubt that we have something like this on the TOS Enterprise,
ideally located between the sports section of the ship and sickbay.
In a certain manner of speaking it's not too dissimilar from the way the TNG holodecks work. Which reminds me that the "Acapulco diving" holodeck program (mentioned in "Conundrum") would require a rather tall holodeck.