As of now, HIMYM shows no signs of really advancing with the stories of each of the characters, and as such, the plots are becoming drab and repetitive. Zoey is quite a disastrously uninteresting and infuriating incarnation to Ted's 'skanks' that it was far too obvious at this point where this plot will go.
This episode is more focused on the psychological repercussions for Ted as he begins to get closer with Zoey. The Captain is a somewhat entertaining character in this episode, played very well by Kyle MacLachlan. As one of the above users has mentioned, the 'less is more' decision would have paid off as better, as it is far too obvious from the first moment that The Captain would not try to kill Ted. As such, it becomes a bit slow after a while and I imagine, does not calculate to high in terms of re-watchability.
Future Ted, from 2030, has thus far been a plot device. A very obvious one at that, but Bob Saget as the voice has been terrific, showing tremendous ability to inject some humour in his very limited voice- time in episodes. At last the show tries something new, which is it gives Future Ted as much of a plot as it gives any of the other characters: he forgets a part of the story. HIMYM is a fairly ridiculous show, partly because of the setting: a grown Ted reciting his life story to his kids. How he remembers everything is obviously stupid, and this episode, acknowledges it to a fairly effective degree. Unfortunately, as many others have said, the voice acting is disappointing because it does not seem to be Bob Saget in his usual role. That said, the story works very well, as a means of showing glimpses of what is to come.
The Marshall and Robin story does not work all that well. It starts off as decent, after a very awkward start to their dinner, but things just get less interesting as it goes on. The titular element of the story, The Mermaid Theory, is actually not that interesting, and even the extraordinary Neil Patrick Harris cannot really do much about it.
This episode is okay, but it does provide some excellent 2030 Ted moments. The Captain, whilst entertaining, could have been a subdued presence in this episode, as far as screen time is concerned, to make for a more effective and enjoyable story. "The Mermaid Theory" still is not that great, and HIMYM crawls along at its typical snail pace.