It looks like some fish are (sort of) warm-blooded. From How Hot Tuna (and Some Sharks) Stay Warm:
Scientists now have direct evidence that the north Pacific salmon shark maintains its red muscle (RM) at 68-86 degrees Fahrenheit (F), much warmer than the 47 F water in which it lives. The elevated muscle temperature presumably helps the salmon shark survive the cold waters of the north Pacific and take advantage of the abundant food supply there. The heat also appears to factor into the fish’s impressive swimming ability.
Salmon sharks are lamnids, a group of sharks that also includes the mako and great white. Numerous studies have shown that lamnid sharks and tunas share many anatomical and physiological specializations that endow them with their impressive swimming power and speed. In contrast to other fish where the RM is near the skin, the RM of these sharks and tunas is near the backbone. Even though the ancestors of bony tuna and cartilaginous sharks diverged more than 400 million years ago, selection pressure for high-performance swimming in each group seems to have occurred independently about 50 million years ago.
Throughout its life, a salmon shark never stops swimming because it will sink. The body heat generated from continuous swimming elevates the RM temperature, which in turn, warms the surrounding white muscle and allows the shark to survive the frigid waters of the north Pacific. If a shark stops swimming, it could die from cold exposure.