Detecting fluid on the male pelvis is actually quite difficult, though once detected it can be there for a number of reasons but it must be said that pelvic fluid is a far more common complaint for females due to reproductive complications.
One of the causes of the fluid could be a pelvic and intra-abdominal injury but you would be aware if you had suffered a trauma to the pelvic region. This can generate free fluid on the pelvis.
Another cause of the fluid is ascites. This affects both men and women and is an accumulation of fluid inside the peritoneum. It has many causes, notably hepatitis, cirrhosis, blood clotting inside the veins of the liver, and/or liver cancer. Again, it would be noticeable because you have a back drop of ongoing illness.
This is an inflammation of the pancreas. The pancreas helps in the breakdown and digestion of foods by secreting enzymes, and, importantly, produces hormones such as insulin.
This results from damage to the kidneys. As a result, they leak large amounts of protein into the blood and surrounding tissues swell as a result. These leaks are from small pores in the kidney too small to admit cells, so there is no blood in the fluid.
This is the inability of the heart to provide blood to the body and one of the symptoms is a buildup of fluid. This can lead directly to a cardiac arrest, which can obviously be fatal so medical attention should be sought if you suspect this.
This varies for each condition and for the underlying cause. Generally speaking, diuretics are commonly used, and antibiotics are always recommended if there is an infection in the fluid.
In any case, a visit to the doctor for tests is highly recommended, given the range and spread of the diseases and conditions that may underlie this condition.