The thing is, yes you can swim forward in this tail once you're in the water ("anything else is a challenge", I believe she said somewhere), but for all the rest that would be needed, like getting in and out and such, you would need a crew of helpers.

I also noticed that swimming scenes in H2O or Mako usually are only a few seconds long.
Not sure if this is due to the actors all being not the best swimmers or the lacking manoeverability of the tails, but I'm pretty sure this kind of tail could not be used without a back-up crew for lifting and safety.

Swimming in an extended tail is incredibly draining and difficult. (Done it myself, though mine isn't silicone so more drag).
Props to the stunt actress for pulling that 25m dive in one.