readPixel Issue on pre-processed S3 L1B OLCI product

Could someone please assist me in clipping an area of interest from a SAFE file downloaded from the Copernicus Browser using a shapefile, and exporting the data of the selected area into an XML file for further index calculations in Excel?

One option is to use Land/Sea mask (Raster/Masks/LandSea Mask) and use a vector as mask.