You can think of a freshwater fish as a leaky boat. The gills being the leak and the kidneys as the pumping system to control it. Relatively speaking the leak is very extensive. Many freshwater fish produce more than 20 percent of their body weight in urine per day, while we will only produce up to 1.5 percent. The major function of the kidney being to rid the body of water, while the gills uptake Na+ and Cl- against a concentration gradient.
Saltwater fish have the opposite problem, they need to drink water because it is constantly diffusing out of their bodies. Unfourunately this also increases the salt inside their bodies. They will only produce 1/10 to 1/20 that of freshwater fishes. Their kidneys main function is to excret magnesium sulfate, with their gills being the major excretory pathway actively transporting excess NA+ and Cl- from the blood to the water.
In short yes they do pee; Freshwater fish constantly, Saltwater fish hardly at all.
And that bluegill thing is because you squeeze their kidney while removing the hook. The kidney runs from the opercules behind the lateral line and dips down to connect with the bladder above the anus. Besides its very dillute.
