Most of my ideas have already been suggested, so let's go with Fluid (PS1).
Hopefully it wasn't mentioned here earlier xD
I would recommend it for fans of electronic music, especially those who listen to ambient and breaks. It's an aquatic themed music maker. Think of Music 2000 but more abstract. It's a good distraction and you can make decent songs with it.
I'm almost certain that I read about another project by the same devs, but I don't recall any information.
I'm aware of other aquatic games for PS1 and PS2 but I still need to check them out.
Things like Aquanaut's Holiday, Everblue, Treasures of the Deep...