Tropical-Rouge! Precure

both are magical girl anime with mermaids! 

Both series are about cute magical girls which main motifs are tropical and ocean themes, and mermaids.  

There's a very good reason why these two casts in particular had a crossover movie: both seasons are much more notably cartoonish and silly than other precure seasons (when they want to be). Both have very goofy casts, memorable villians, and similiar enough pacing. All aside, I think anyone who likes one should check out the other. 

Both are more absurdist seasons that throw the zany at you nearly every episode. Five leads whose differing personalities playfully bounce off one another, colourful locations for some enjoyable battles, and antagonists that are just trying to do their best. 

Also known as Balala Ocean Magic, well, the title says it all! Tropical Rouge Precure has a less obvious, but still relevant title. Don't get it yet? Let me spell it out then - both series are about magical girls who deal with ocean themes! This Precure series is more slapstick comedy than usual, so the feel and comedy of each is quite similar, along with the theme, genre, and animation. Balala did the ocean theme first, but Precure is the more well-known franchise. Precure focuses more on action and is more character-driven, while Balala has a more slice-of-life feel and is story-driven, though each has plenty of cute girls doing cute things! read more 

The two shows take place near the beach and focus on a club of girls. The protags are very similar and meet a girl on the beach and yeah.