I think there was a lot of disagreement all around on whether Mars had oceans or not. The evidence was maddeningly contradictory. The question also has been the longevity of any Martian oceans. For now the evidence appears pretty solid there were oceans for a long time, and not just massive short lived floods.

As far as I know there is no conclusive proof as of yet that the asteroid belt was once a planet.

Ben Bova's "Jupiter" theorizes a massive global water ocean that exists as a layer around Jupiter hundreds of miles below the clouds.

The last big solar storm blew a big piece of Mar's atmosphere off into space. No magnetosphere is a big problem.