As this subject seems to come up often, I thought that a letter to the editor of Popular Woodworking magazine, page 16 of the June 2003 issue, might be worth noting. The question was about putting water based poly over a tung oil/varnish finish. The question was answered by Bob Flexner, contributing editor, who said that the tung oil/varnish was very slow drying and the floor would be unusable for three days per coat. He also noted that oil based poly would wear a lot longer than water based poly.
If you are thinking about laying a hardwood floor in the kitchen, you might want to check this out. Many libraries get this magazine.