No fish is really warm blooded like we mammals are. The mackerel sharks( Mako, Great White, etc. ) use metabolic heat to raise their temperatures up to 25 degrees F more than the temp. of the water. Tunas use the same "technology" as well. They have red heat producing muscles under all the white muscles.
The reason these fish have these features is because warmer muscles move faster and work harder, hence why the tuna is one of the fastest fish in the ocean. The mackerel sharks on the other hand use this metabolic heat for stamina. Sharks( well most sharks anyway... nurse sharks do not need to move to breathe ) need to always be moving to breathe. They do not have muscles on their gills like normal fish so they are always in constant movement to pass water over their gill slits. If a Great White shark stops moving it will drown. They even sleep while they swim. This is why sharks need the extra heat, so their muscles can keep going all the time.
The sharks have small viens in their muscles which pass next to small arteries. The veins carry warm unoxygenated blood to the gill slits from the muscles. The atreries then carry cold oxygenated blood back to the muscles. These atreries and veins are very close to one another. They are called the "Rete Mirable". The warm blood carried by the veins warms the colder oxygenated blood going to the muscles. Warm blood means warm muscles which means better operation. I hope this helps.
PS- There is no such thing as a FW warm blooded fish. All fish are cold blooded, other than the ones mentioned above, which are not really warm blooded they just tricked nature. They cannot hold a constant temperature so they are not considered warm blooded.