I haven't met exactly same issue, but lost watch list in data viewer on Windows some times.
They are saved in registry on Windows, so it may be saved in .plist file in your Mac.
Apparently, there are no .plist files. I tried to trash the folder named "FileMaker" in the "Application Support" folder in the Library under OS X, but it didn't help. When I put in new hosts, later on they were gone. I have stopped using FileMaker 14 completely. I took it out of my applications folder, put it into the trash, and emptied the trash. I'm back to 13 until this is resolved. The reason is, when 14 erases all the hosts, it also erases them all from 12 and 13 as well. I have nothing left to refer to. I'm not going through this again, not until someone tells me where the data for these is really stored and why this is happening! So far, in searching this database, there is no information on that.
(~ mean users folder)
There does not appear to be such a file on my Mac. I think I looked there before and found nothing. I did find the correct folder (Preferences) just now, but nothing with "com.filemaker ... " or anything like that is in there. There are a number of other similarly named files, so I know I'm in the right place. I'm wondering if Apple or FileMaker changed things when going to OS X 10.10 (Yosemite) and it's no longer stored that way or in that folder.