How dangerous is excessive fluid retention and abdominal swelling?

It can be. Excessive fluid retention can be the result of a benign process such the result of a diet resulting in a lot of gasseous abdominal distention or fluid retention due to excessive salt. However, it can also be due to serious medical conditions that effect fluid balance such as heart failure, liver failure, kidney failure. Although rare in this country, severe protein calorie malnutrition is a caus.
Depends upon extent. Can compromise breathing. Can cause reflux can interfere with venous return if causes liver failure.