An Update ...
Work on Grib2 using the grib_api has stopped.
Grib Version 1 tidal currents are no longer available from Weather4D unless you have their app.
Weather4D sources this data from the EU CMEMS project
(Copernicus), where it is freely available as NetCDF (nc) format files. These cover a large area of Western Europe
and are potentially more useful than the BSH Grib 2 files.
Downside is the long-winded download process from Copernicus, although there is a Python script that can be used for this.
I am now using the nohal/konni Grib2 plugin as a template for reading the nc files downloaded from Copernicus. The code uses the NetCDF library from the Center for Marine and Atmospheric Sciences
There is some discussion in the background about whether a separate plugin is needed or whether the capability to read nc could be added to the Grib plugin. For this reason work on the Grib2 plugin (for nc files) has been suspended. The GitHub Grib2 has been left as an example of reading nc. I am now looking at ways of adding nc reading to the Grib plugin (With very slow progress!!!).