"The Ambergris Element" ***
A water world being studied holds a secret that could radically change the lives of Kirk and Spock.
There are a number of things going on in this story, some of it familiar and some novel. The familiar is the idea of the Enterprise
crew being caught between factions on an alien world. The novel part is having Kirk and Spock changed into something more closely resembling the inhabitants of that world---specifically: Kirk and Spock are medically altered into water breathers. The story basically plays with an idea already popularized with the likes of DC Comics' Aquaman
and Marvel Comics' The Submariner
as well as later in The Man From Atlantis.
Indeed there's a good dose of the idea behind the ancient myth of Atlantis mixed into this, only here the sunken civilization is set on a far off alien world.
It's not a bad story idea, but it's hurt by two main things: the story feels rushed and truncated and there is too much visual shorthand at work. By "visual shorthand" I mean things like Kirk and Spock continuing the whole episode in their duty uniforms particularly after they've been so radical altered. Also seeing seeing Scotty descend underwater in uniform and protected solely by a life support belt was just a bit too WTF for my tastes. Add to that seeing McCoy and Chapel as well as Scotty in uniform while in the temporary sickbay tank needed for Kirk and Spock while they're in altered form.
In a broad sense this redresses familiar ideas and stories used in TOS episodes such as "For The World Is Hollow And I have Touched The Sky" and "Return Of The Archons" and others. TAS' story is sufficiently redressed to appear novel enough even with the reuse of familiar ideas, but again I feel it's lack of nuance in story and visuals that keep the episode just okay rather than allowing it to be rated as good.
Two things I really do like in this episode: the concept of the aquashuttle and the design of the Argoan sea serpent.