I have been trying to download multiple sentinel-3 products, then unzip and attempt to load them into SNAP.
Each time I get the following error in SNAP:
Based on this comment: Sentinel 3 open error, not all sentinel-3 products can be opened in SNAP. But what sentinel-3 products, that are monitoring land conditions, can I open in SNAP?
Turns out that I can only use the products that can be downloaded via the Sentinel-3 Pre-Operations Data Hub: https://scihub.copernicus.eu/s3/#/home
and non from the Copernicus Open Access Hub: https://scihub.copernicus.eu/dhus/#/home
I think this will change in the future, so my solution will only be valid for a limited timeframe.
On the Copernicus Open Access Hub are only the altimetry products available. These are not supported by SNAP and the S3-Toolbox.
For the altimetry products please have a look at http://dedop.org/ and http://www.altimetry.info/toolbox/