No marine animal is a mammal capable of true sustained flight; bats are the only mammals capable of powered flight.
Several ocean animals appear to fly, but they do so by gliding or leaping rather than by powered wingbeats. Flying fish launch from the water and glide using enlarged fins. Mobulid rays can leap above the surface, but they cannot fly through the air. Seabirds, meanwhile, are marine animals that truly fly, but they are birds rather than mammals.
Among mammals, bats are unique because their wings are modified forelimbs that generate powered lift. Marine mammals such as dolphins, seals, whales, and sea otters are adapted for swimming, not aerial flight.
This distinction matters because “flight” can refer loosely to any airborne movement. In zoology, true powered flight requires repeated active movement that produces lift and thrust. Gliding, jumping, and being carried by wind do not meet that definition.