Steroids cause water retention both outside and inside muscles.
Yes you can take diuretics during cycle, many do. Either for edema or high blood pressure or both. But you have to know what you're doing. Some use HCTZ, some may use Lasix but it depends on your situation which would be appropriate. I don't think you should self medicate with diuretics. Work with a doc if you need it.
Some steroids make you look leaner and have less water retention due to an anti-mineralcorticoid effect. I think tren is one of them.