The ocean is the place of origin for all life. Especially in Jungian interpretation, this is a place of creativity, fertility, and birth. There is also a lingering sense humans have of rising up out of the collective origins of life in the oceans. It is easy to see the Darwinian influence of these ideas. More importantly, it is easy to see the lack of connection some dreamers could have with this approach to dream interpretation.
Given the widespread popularity of boating, scuba diving, and cruise vacations, numerous people have experiences with the ocean that were not available in the past. It may be that the dreamer has one of these connections to the ocean, rather than a general perception of fertility.
For some, the ocean can impart a sense of fear and foreboding, especially if they can't swim. Its ultimate vastness, coupled with their lack of swimming ability, can appear in a dream as a reflection of some insurmountable struggle they may be having in waking life.