I wrote "zombie" in quotation marks, because for the most part he wasn't mindless. But I'm getting ahead of myself.
An American film I saw somewhere mid-90s (might be tad older, of course. 80s?). In color and very bright, as if it took place in LA or somewhere else in California. Or maybe Florida. Somewhere in USA where swimming pools by the houses are common, because there definitely is a scene around a swimming pool.
It's a buddy cop movie about two cops (or detectives?), both white male, but it has a supernatural twist: in their investigation they discover some serum? or technology? that reanimates the dead. Or prevent death, make a person immortal? Something like that. They discover how it works because in the film one of the cop partners actually dies (is drowned, I think, but not sure) but he remains alive and active. Not brain dead like a zombie - he's still fully doing his job as a cop. I remember him discovering that when he cuts himself, he doesn't bleed.
There's also some lady helping them in the investigation. Not a cop. Scientist, maybe?
Only at some point later in the film that "zombie" cop really dies (I think maybe it's at this point where he's drowned - I vaguely remember his head in a jar of water?) and when is reanimated, he's mindless and tries to kill his partner and the lady. I think they manage to get to him to remember who he was.
That's all I remember. Will be very thankful for your help.