Smartest creatures around in the sea?
There are over 800 species of cephalopod, although new species continue to be described.
Cephalopods are found in all the oceans of Earth. Cephalopods are thought to be unable to live in freshwater due to multiple biochemical constraints, and in their +400 million year existence have never ventured into fully freshwater habitats. Cephalopods occupy most of the depth of the ocean, from the abyssal plain to the sea surface. Their diversity is greatest near the equator and decreases towards the poles.
Cephalopods are widely regarded as the most intelligent of the invertebrates, and have well developed senses and large brains. The nervous system of cephalopods is the most complex of the invertebrates. Cephalopods are escape artists, have been known to climb out of their aquaria, maneuver a distance of the lab floor, enter another aquarium to feed on the crabs, and return to their own aquarium.
They are also social creatures; when isolated from their own kind, they will sometimes shoal with fish. Cephalopods have advanced vision, can detect gravity with statocysts, and have a variety of chemical sense organs. Octopuses use their arms to explore their environment and can use them for depth perception. (Wikipedia)