It happens with such regulatory in Thai BL, I suspect that a particular aquarium is rather like a house in my grandmother's neighborhood in Pasadena. Though it may have burnt down since, the house was one of the places our family would walk past when we visited in my childhood. Someone was nearly always filming there, usually scenes that were supposed to take place in New England (the house has/had that look) complete with fake snow on the ground.
Interestingly, this use has passed from "hey, the aquarium lets us film on their premises!" to a standard date for BL characters, even in manga. That is, the aquarium date has become a trope, much like amusement rides and haunted houses! In the Japanese BL series, Stay by My Side After the Rain, the characters visit an aquarium for their first date after they meet again (since high school).
There is something quite inviting about fish. And the set-up is perfect for action-while-talking. Sitting in a theater is too static. Dancing is too noisy. The aquarium is perfect!
The below is from one of my favorite short manga stories: "Lover's Pledge" by Kae Maurya. The nervous main character and alpha Darcy-like character get to know each other on "innocent" dates.
