cannot open S6 eumetcast files in snap 8.
I uncompressed the raw .tar eumetcast file into the folder
S6A_P4_2__HR______20220614T232801_20220614T233801_20220615T011508_0600_058_234_117_EUM__OPE_NR_F06.SEN6
but snap does not open the xfdumanifest.xml
SNAP is not supporting Sentinel-6.
For altimetry there is a separate toolbox available.
Radar Altimetry Tutorial and Toolbox
But I think it is also not yet supporting S6 data.
Maybe you can work with the netcdf files contained in the SEN6 folder?