No Evidence shown for vast oceans on ancient Mars
John Dohm is a nice guy, but not one of the world's most incisive thinkers and most of his work is about what "feels right" to him. There is certainly no evidence presented in that brief discussion of the actual amount and distribution of thorium (and other elemnt) enrichment.
Anyway, there are at least a dozen mechanisms that could produce altitude-dependend distributions of these elements and only some of them involve water.