I have the same phone for the same reasons (and being waterproof) but it definitely doesn't last 2 days anymore. 25 hours is probably my limit, with my reasonable amount of usage.
If I'm in an area of good service (i.e. not my cement and steel coffin of workplace), my Z3c lasts 48-60 hours -- 3 "days" of use -- with ~5 hours of screen on time. I don't do much gaming on it, which probably helps.
When I'm at work though, poor reception brings me down to 30-36 hours, still pretty close to 2 days of usage though
When I'm at work though, poor reception brings me down to 30-36 hours, still pretty close to 2 days of usage though