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This episode is based off of the events in Chapter 50 of the manga.
The opening lines are more or less the same. However, in the manga, Megumi wasn't the means of getting Keiichi to the beach; it was Urd and her fortune-telling. The argument between Urd and Skuld over the television didn't exist in the manga. The television fight could be a reference to chapters 128 and 129, which focus on that.
The Megumi connection could be alluding to the OVA, though. The OVA's second episode had Megumi suggesting to Keiichi that he take Belldandy to the beach.
Megumi coming over for breakfast every morning is taken from chapter 16.
But, basically, the whole Megumi conversation is new material. As I said before, the flashback and circumstances were between Keiichi and Urd.
The sequence with the Blue 6 is altered slightly. Keiichi tries to mention that he can't swim, but he actually does get to say it in the anime. Skuld's not being able to swim is new to the anime.
Originally, when Keiichi comes to, they're still on the beach. The events here change significantly. Originally, they take a walk on the beach since Keiichi's looking for the Three Sisters Rock (and failed), and Belldandy recalls that their very first date was by the ocean too. They then come back to find Urd topless, and Urd tells him that he needs to look again.
Basically, the eavesdropping, interrupted kiss, and bath scenes are new. The fortune-telling originally took place at the very beginning of the story, not right now either. The fortune-telling didn't involve sea cucumbers either.
The scene with Skuld preparing the traps and Urd's monolouge supporting Keiichi is kept. The scene between Keiichi and Belldandy is more or less the same, as is Keiichi rushing out to find the rocks. The traps... are the same.
Heck with it, the rest of the episode is as faithful of an adaption as all-get-out. Except for the ending. Megumi didn't show up, and there wasn't any gloominess. Urd telling of it being a joke is kept, but Belldandy's happiness and reaction is new to the anime.