Marine mammals are mammals that are reliant on marine resources for at least some part of their lives, and is a term generally used to encompass a variety of carnivores including polar bears, sea otters, seals, sealions and walruses ("pinnipeds"), and whales, dolphins, and porpoise ("cetaceans"). The cetacean group includes the toothed species (odontocetes) which have rows of peg-like teeth and prey on fairly large marine animals like fishes and squid, and the baleen whales (balaenopterids) that have baleen plates that they use like a sieve to scoop up smaller zooplankton (e.g., krill) and fishes.
Goats are definitely not marine mammals in any traditional sense, but I'm giving the feral goats on Jura a special mention because the population there has adapted to rely heavily on seaweeds as part of their diet.
Large pelagic fishes are not seen from the surface very often (as you'd probably imagine), so they're included in this gallery for now until I eventually take enough photos to warrant giving them their own space, but don't be confused: dolphins and whales are certainly not fishes! There are a lot of ways to tell the difference, but if an animal is large (< 1m long), has a blowhole to breathe air, and swims by moving its tail (fluke) up and down then it's a mammal. The fishes that are large enough to be confused for a whale or dolphin all swim by moving their tails sideways.