Evolution of Intelligent Life... Underwater?

Details:

1. "The ocean covers more than 70% of the planet's surface" (NOAA).
2. "95% of the ocean remains unexplored, unseen by human eyes" (NOAA).
3. Microbes and oceanic species has been existing before any intelligent mammals, such as humans (Wikipedia).

With this, how probable is it for a intelligent, or even highly sentient, being to exist underwater?

By intelligence I mean "the tendency toward control of the environment -- including a non-hereditary adaptive quality developed during the lifetime of a single individual (that is, intelligent creatures can learn)" (NASA).

Sources:

NOAA- http://oceanservice.noaa.gov/facts/exploration.html

Wikipedia- http://en.wikipedia.org/wiki/Timeline_of_evolution

NASA- http://astrobiology.nasa.gov/ask-an-astrobiologist/question/?id=918