As of SNAP 11, EUMETSAT OLCI L2 water products that contain band-by-band uncertainties (_unc) do not work in the spectrum viewer. The viewer reads the reflectance uncertainties as the band set and provides no default option for adding the reflectances themselves (though this may be possible with bespoke band sets). The problem does not occur with SNAP 10, or with older L2 products that contained a folder of band-by-band errors (_err).
SNAP 11 throws an error when using the spectrum viewer with these products, which appears to be related to variable typing and/or bounds. I can supply the error if required, but have reverted to SNAP 10 for now! I am running on Mac OSx Sonoma.