It completely depends upon who the actual artist is (Say Rembrandt for example) and how rare the watercolor painting is.
For a current non-famous artist, it would likely depend upon the subject matter of the painting and what market it was being sold in. Here’s a poor example but it makes the point: If an artist were selling watercolors of particular touristy beach (at that beach) he could probably sell them for more than he could some other oil paintings of random places. In other words, he would be marketing the beach watercolors to that particular market. And even if he is a fabulous oil painter, the target market is the tourists who want the beach paintings.
And the size of the painting (if the watercolor is bigger than the oil painting) it would probably sell for more.