It is unclear whether dolphins have tastebuds in their tongue, because they swallow their food whole, but there have been cases when individual dolphins have demonstrated preferences for certain species of fish. It is also suspected that dolphins are able to communicate by excreting chemicals, called pheromones, for others to taste.
In order to maintain water balance, marine fish undergo a process called osmoregulation. Fish must counter these changes to maintain homeostasis: 1. Drinks sea water 2. Minimal urine produced (and so it is very concentrated). Kidneys lack glomeruli. Tubules actively secrete MgSO4 3. Gill membranes are relatively impermeable to water. 4. Gills actively secrete sodium from chloride cells; chloride ions follow. Source: Nature of Biology Textbook - Book One - Fourth Edition
Whales
Whales spend all of their lives in water, but because they are endothermic, they need to come to the water's surface surface every couple of minutes. In order to maintain their body temperature, they have an insulating layer of fat, called blubber, which is deposited around organs, tissues and in bone in the form of oil. Blubber below the skin can be up to 50 centimetres think. Whales maintain a temperature of 36-37°C in an environment that is typically less than 25°C by using a countercurrent exchange system. When the whale needs to conserve heat, the outermost blood vessels contract, little blood flows and heat loss from these vessels is reduced. Additionally, heat flows from the warm blood coming from the core of the body into the cooler blood that is returning to the body from the skin. This process occurs in the fine vascular tissues of the whale's fins and tail, because this is where heat is more readily lost.
Source: Nature of Biology Textbook - Book One - Fourth Edition