Frogs are amphibians, meaning they need land. The open ocean does not have land.
Dolphins live in and travel around the ocean, not needing land and capable of swimming great depths so they are capable of living in the open ocean.
Alligators live in areas with shallow waters where they can cool down and hide to capture unsuspecting prey. They need to breathe every now and then and even though they are fantastic swimmers in rivers, they can’t go too deep. They also need land to lay their eggs and mate, so the open ocean is not the best option for an alligator.
Hippopotami also live in shallow water like alligators. And like alligators, they need to breathe every now and then and they can’t go too deep in water. Rivers are great for them, but they won’t last too long in an open ocean.
Thus, the organism most likely adapted to the open ocean are dolphins.