...But, they're already at the beach, why not have some fun?
Parodied: The only people on the beach wearing revealing swimsuits are very oldpeople.
There is more than one beach episode in the series, and everyone wears a different swimsuit each time they go to the beach.
Alternatively, the swimsuits each have different levels of conservativeness, with both the male and female cast showing varying amounts of skin. While Bob and Alice have relatively conservative swimsuits, Carol and Daniel's are absolutely skimpy.
The cast never goes to the beach.
The entire cast wear conservative business suits.
The episode was made as straight-up fanservice.
Alternatively, the authors wanted to give the audience some time to relax in between arcs.
Lampshaded: "Remember ladies, the whole gang's going to be at the beach tomorrow, so wear the skimpiest things ya got!"
The female characters deliberately wear revealing swimsuits in order to appeal to the opposite sex.
"Enough saving the world for a while. It's time for a beach break!"
Exploited: Emperor Evulz uses their day at the beach to fast forward his plans while their at the beach.
The female cast insists on wearing conservative clothing at the beach.
Played For Laughs: The female cast members secretly arrange a contest to see who can keep the guys' attention longer — while the guys have their own secret contest to see who can act the least perverted for the longest. The girls spend the whole episode taking increasingly desperate measures to get the boys' attention, who in turn refuse to acknowledge a thing.
Played For Drama: When Bob and his True Companions go to the beach, Shrinking Violet Marianne suffers from A-Cup Angst and weight issues, becomes very depressed while at the beach, watching so many of the other girls in the group wearing revealing outfits and appealing to men. She becomes so unhappy that she drives off in tears and refuses to return to the beach, resulting in a disconnect between her and the rest of the group. Which is bad, because Marianne is one of the most important members.
Be sure to apply enough sunblock to you before returning to Beach Episode