Note: Due to depression/anxiety, I let myself go recently health wise and am in the upper range of healthy bmi for my height, I think I'm overweight but can't judge clearly as I've had issues with anorexic stuff in the past. Also when I last checked, my "bra" size was around a B or a C (no idea of the measurements).
For a regular binder, I double binded to get a "flatter" affect (did this all year round). It's not necessarily the best though, so keep that in mind and be careful. My regular binder (Underworks 997) would "ride up" at the bottom and create a "uniboob" affect up top after a few hours of work. My swim binder (Underworks 918) would keep those two side affects from happening, so I would wear the swim binder over the regular binder each day. I also wore lose shirts so that helped.
As for the swim binder, I think this would depend on how overweight you are and how much you would wear it. For swimming, I did the same as I listed up above and wore both the regular and swim binders. The swim binder alone was great the first few times I wore it but then would stretch out after a while and create the "uniboob" thing again. At least that was my experience. Oh and I also always wore a lose shirt over top of it while in the water (that was the most comfortable for me as I didn't like the "armpit" thing that can happen with binders).
Good luck. And if you buy a swim one and it doesn't work 100%, keep in mind you could always modify it somehow to make it useful for you somehow (maybe as an exercise shirt under a t-shirt, or double fold it on itself for a better binding affect, etc).