Many experts suspect that oceans flooded some regions and that rivers coursed through others. All of which supports the notion that life may once have flourished on the Red Planet (Cowen, 2002)."

The question is no longer whether or not oceans existed on Mars, but is now, where have they gone? There are several theories about what happened to the oceans including the possibility that they have gone underground or over a long period evaporated into the atmosphere.

The final piece of evidence regarding the existence of past oceans on Mars can be found in a study that reported an area near the northern plains indicating sediment that had to be caused by water. Scientists liken it to a bathtub ring in explaining how it points to a large standing body of water, most likely an ocean, was there at one time (Cowen, 2002).

Conclusion

There have been many experts speculating about the possibility of water or oceans on the surface of Mars, but recent ability to identify sediment, flood damage and other elements indicate that past oceans on the planet are a fact.
References

Cowen, Ron (2002) Exploring the Red Planet: Mars Odyssey set to begin its mission.

Science News

____(2001) Martian chronicles; an Odyssey to Mars.(Science and Technology)(the more scientists know about the place, the less they understand it. Will this weekend's launch of a new mission to Mars help?) the Economist (U.S.)

Statter, Charles (2006) (White Mars; Planetary science.(the geology of Mars) the Economist (U.S.)