Director Jeff Lau Chun-Wai is famous for his mo lei tau (literally 'makes no sense') humour, and The Fantastic Water Babes does have some genuinely funny scenes. Excellent cutaway scenes and visual slapstick gags mean there are decent comedic moments, but that's about where the good news ends, as the romance element totally fails.
Gillian and Chi, naturally, are attracted to one another and there is an oddly successful chemistry between the actors, but the poor script makes the duo always seem uncomfortable. Almost every "romantic" line from Fong is massively cringeworthy, Chi is kind of a jerk, and Gillian is whiny and neurotic.
Elements of the film are beyond ridiculous, such as magic - yes, Harry Potter-style - which is thrown in seemingly to show off some truly average special effects. Hit the beach instead of wasting time watching this sinker.