To maintain homeostasis, any excess nitrogen must be disposed of. Three possible ways. Most animals that live in water do get rid of Nitrogen as ammonia, quickly diluted so not a problem. But land-dwellers would quickly poison themselves, need different form. Another major problem for organisms is to regulate osmotic environment.
Saltwater fish excretes excess salt by active transport through gills.
They make much more concentrated urine than we do. No, for two reasons. Our urine is fairly dilute. If try to survive by drinking sea water, will be dead within 3 days from dehydration. Eating fish not good, because loaded with protein, forcing excretion of lots of urea, loss of fluid in urine.