One of the neat features about SharePoint 2013 is that Office Web Apps has been split off into it's "own product" so it doesn't have to be installed on a SharePoint server anymore. There are lots of reasons to do that, like being able to maintain a separate patching schedule, being able to do a patch with no downtime, using the same OWA farm for multiple SharePoint farms or other applications, etc. One of the things that can be tricky to figure out though is what to do when you need to troubleshoot an issue.
Well it turns out that we still have ULS logs for OWA, even though it's not "SharePoint" per se. The log files are not where you're used to finding them though - you need to look in the C:\ProgramData\Microsoft\OfficeWebApps\Data\Logs\ULS folder to find them. That's the default location, you can change it as well using PowerShell. Once you find them the same tools that you use to examine SharePoint ULS log files will work with these as well. Hopefully you won't ever need to use this post :-) but just in case, here it is.