Of course, strictly speaking, divers are not 'sealife', but scuba enables us to come closer to being aquatic than most mammals - and certainly closer than any other primate.
Size is a fundamental quality of organisms.
The marine environment affords unique opportunities and challenges for living creatures. How successfully a creature exploits the resources of the sea depends in part on being the 'right' size. Small creatures can absorb dissolved oxygen directly through the body surface, but low oxygen concentration places firm limitations on how large a gill-breathing animal can grow. With these factors in mind, let's consider the size range of marine creatures. This is not an easy problem.