Both dramas have a female lead who is a fan of the ML, saving the ML from their future death and both ML are stars/singers.

Both are about catching criminals and have a detective with some sort of power, but in Higanbana; Keishichou Sosa Nana Ka?, she sees the memories of the crime scene rather than the memories of the person

-Bromance
-School Setting
-Swimming
-Cute Romance

I feel like A love so beautiful has a similar setting to My Mr.Mermaid. They both have swimming involved . Though A love so beautiful is more focused around the school aspect whereas My Mr.Mermaid is more focused around the swimming aspect