Both deca and test are known to cause a lot of waterretention - yes, but the water retention will be minimal if you are using the proper support supplements with your cycle:
1. An AI to keep estrogen in the low-normal range. Aromasin at 10mg EOD is a good dosage.
2. An anti-prolactin drug to keep prolactin in the low-normal range. Cabergoline at 0.25mg EOD is a good dosage.
High estrogen and prolactin levels can lead to side effects like gyno and waterretention. If you add in those two support supplements, the waterretention will go away in a couple of days. Ideally you would have been using them since you started your cycle but better late then never.
The best thing you can do to prevent water retention is to drink a lot of water. Water is your body's most precious resource. You can live months without food but only 3 days without water. If you are dehydrated, your body will hold onto water and retain it as a defence mechanism. Drink 4-5L (minimum) of clean water each day.
On top of that, watch your sodium intake (don't eliminate it completely).
Any unprocessed carb (e.g. oatmeal, brown rice, sweet potato etc) will have no sodium at all. It's only processed carbs like boxed cereals that contain sodium.