The Martian oceans may yet prove to be a mirage. Scientists analyzing the Martian data may have had a case of mistaken hematite identity:

Although the NASA rover Opportunity has found other evidence that the plain was likely to have been a shallow sea, it has yet to find a single flake of the grey hematite.

"It's not panned out so far in the images we're seeing," said U.S. geochemist Professor Donald Burt of Arizona State University.

Opportunity may be finding the same old red hematite that gives the planet its nickname, eats away at our cars here on Earth and doesn't require nearly as much water to form.

I'm still more interested in the methane, myself.

This is not my field - but based on what I saw at March's LPSC meeting, it seems this story reflects some unhappiness, on the part of some members of the community, about how the thermal spectral data has been released. There was some partisan clapping - and some shaking of heads - at one talk in particular. Papers and e-mails had been angrily flying about prior to the landings.

However, hematite has a spherical crystal habit which would be compatible with the so-called 'blueberries', and would provide an alternative explanation to one Mars Global Surveyor observation that had been used to predict large 'flakes' of hematite. The independent Mössbauer spectrometer observations from MER imply 'well crystalized' i.e. grey, hematite, and that it was concentrated in the 'blueberries'.

We will see what happens when all the rover science goes through peer-review.

There is quite a bit more evidence for a salty sea at the Opportunity site - salts, bromine, chlorine, sedimentary layers with inclusions, etc. Still, I'm not quite sure what you mean by a "waterworld." Mars has quite a lot of water, probably once had much more. How much was there, whether most was ice, underground, liquid under surface ice, exposed, etc. are still open questions.

My guess is that the methane is volcanic, but there are several possibilities. These are perfect examples of how these probes answer some questions but dangle many wonderful new questions under our noses. Ain't it great? We need to take a better look!