Error opening sentinel-3 products in SNAP

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:
and non from the Copernicus Open Access Hub:
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 and