Dolphins don't, to our knowledge, indulge in self-destructive behaviour. They don't hurt or kill each other for kicks. They don't knowingly destroy the only habitat they've got. They don't spend their entire lives trying to figure out what someone else wants them to do, and doing it (unless they're unfortunate enough to be captive). Smart is as smart does.
A group of scientists in Florida earlier this year showed that the communication between dolphins increases when they are undertaking a difficult task - in one case removing the lid from a canister - as if they were discussing the best solution. Scientists already knew that dolphins use more than one thousand different types of whistle depending on social context but it was unclear if they could communicate directly with each other, one to one.
All creatures have a way they communicate with each other, that is basic among all animals and dolphins are not an exception only that that they are more intelligent compared to other animals. Even so, that does not make them smarter than human, humans are the smartest in all creation.
My argument is from a religious viewpoint and I personally believe that human beings are the smartest on earth. That dolphins might be able to do things differently from humans does not make them smarter than humans. Dolphins and humans were not made for the same purpose, so one cannot necessarily be smarter than the other.
I'm sure that dolphins are very intelligent creatures, and of course they have a method of communicating with one another. However, this certainly does not mean that they are as smart as or smarter than humans. Humans are a unique part of creation, and animals are just not the same.