Shift the borehole (negative depth?)

I have a few soil borings performed in open water. The depths recorded on the field log are from the mudline, and all associated lab testing is referencing depth from the mudline. Is there a quick way to show say 10 ft. of water above the mudline without having to manually edit all the lab data?

I tried negative depth in the sample list and in the description, but nothing is working. Any suggestions?