This episode is a bird lovers delight, as the Calypso crew heads to an island off the Mexican coast. They are there to investigate seabirds and this place is chock full of 'em! The four types of birds they particularly focus on are Frigate birds, the Blue-footed Booby, the Brown Booby and Tropic birds. They cover the island and it is interesting. Among the interesting things you learn is the very long incubation period of Frigate bird eggs and that the young stay with their parents a year--which is incredibly unusual for a bird. You also see Mexican folks harvesting feathers from the Boobies...sometimes by unnecessarily killing them.
There is a big problem with all this, however. Despite the word 'undersea' in the title of the episode, not once do you see anyone don diving gear and there is almost nothing undersea about it other than one brief clip of the birds plunging into the water to feed. It's because of that I am giving this one a relatively low score-- it's just not what fans of the series would be looking for, though I did mildly enjoy the birds.
FYI--One thing I noticed about the show is that they talked about the Brown pelican and how it was endangered. Well, here in Florida they are EVERYWHERE and are a great story about a species that has rebounded very, very well since the 1970s.