Water retention may be caused by either kidney disease, heart disease or hypothyroidism (most probably a combination of the three).
The most common cause of chronic kidney disease is diabetes, but atherosclerorosis is a likely cause too, and will also account for heart disease too. Modifiable risk factors for atherosclerosis are high blood pressure, high cholesterol, smoking and diabetes (I say ''modifiable'' as age, race, sex etc cannot be changed).
High blood pressure can also cause chronic kidney disease which can cause high blood pressure, so it's a bit of a vicious cycle. Diuretics may be of benefit in both reducing blood pressure and off-loading some oedema.