Major body of water (5)
(Other definitions for sea that I've seen before include "The Med, e.g." , "The tide" , "70% of the earth's surface" , "Great expanse of water" , "Cause of flooding" .)
(Other definitions for ocean that I've seen before include "A mighty sea" , "Sea - canoe (anag)" , "Large amount of water and" , "eg Indian, Pacific" , "It's pretty wet!" .)