There are number of things that can cause water retention. My dad has had three rounds of fluid build up and swelling during this cancer. With him, there is involvement with his liver. The liver produces proteins that among other things help to hold fluid in your vessels and keep it from seeping out. So, I think that's what he might be dealing with. The diuretics do help when they are prescribed. The second time around, the Lasix didn't do the trick by itself, so they increased the dosage and added Zaroxolyn. We're waiting to hear what they will decide to do this time.
Your doctor is the best person to ask. Maybe they can adjust the meds.