Many of the extensively surveyed geological features of Mars suggest that the planet once had an extensive liquid water ocean.

The theoretical ex-ocean has been named as Paleo-Ocean and Oceanus Boreali, and some researchers suggest that it could have covered up to 30% of the planet's surface. The concept, however, is not yet universally accepted.

The existence of a primordial Martian ocean remains controversial among scientists. The Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment (HiRISE) has discovered large boulders on the site of the ancient seabed, which should contain only fine sediment. However, the boulders could have been dropped by icebergs, a process common on Earth. The interpretations of some features as ancient shorelines has been challenged.
Alternate theories for the creation of surface gullies and channels include wind erosion, liquid carbon dioxide, and liquid methane.
Confirmation or refutation of the Mars ocean hypothesis awaits additional observational evidence from future Mars missions.

Source : Wikipedia

If there was an ocean, there is an additional problem regarding the fate of the 6 x 10⁷ km³ of water which it's estimated to have contained. Some research teams suggest that most of it now remains underground, or that it evaporated into the atmosphere and was subsequently lost into space.

Also see : Mars - methaneplugin-autotooltip__plain plugin-autotooltip_bigMars - methane

In 2004, three different research groups independently announced the discovery of significant concentrations of methane in the atmosphere of Mars. But the predicted lifetime of methane in the Martian atmosphere has been calculated at just 200 days (Source :