A lot depends on water temperature. Dolphin calves drink milk from their mothers just like humans do. When they are about six months old they start eating fish. They probably learn this by watching their mothers feed. Lots of people think that dolphins are really smart but scientists don't have any information to prove it.
Most dolphin biologists think that dolphins are about as smart as dogs and chimps.
You said that we don't know how intelligent dolphins are because we can't give them tests. The Kewalo Basin Marine Mammal Laboratory does cognitive studies with dolphins that have proven they are extremely intelligent. They also have done extensive studies in echolocation. Most whales and dolphins are not endangered. Those that are endangered have become so because humans have destroyed their habitat or affected their habitat in other ways.