You may be interested in era5 data for historical reanalysis. This is published by the ECMWF and is available for free. I’m pretty sure it has solar irradiance data but I’m not certain. You can also get NCAR reanalysis and other model reanalysis products through the NWS[2].

For visualizing gridded data products you can use panoply[3]. If you just care about GRIBS zyGrib works pretty well too.

[1]https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datas... [2]https://www.ready.noaa.gov/archives.php [3]https://www.giss.nasa.gov/tools/panoply/

There is a startup in Norway called Glint Solar that may help. They serve the floating solar industry with geospatial tools and data, both on reservoirs and offshore. If you can expand a bit on your questions, there may be more resources to point to.