I guess you already know how to sail/boat habe paper charts on board and just want a convenient way to get updated digital maps.

You could try https://map.openseamap.org/ but it will be limited regarding the information you get.

I tried pirating the training maps for the „Sportbootführerschein See“ (German drivers license for boats in German waters) and had no luck.

I would not bother pirating this for two reasons:

1. The only valid maps (for German waters) are made by the „Bundesamt für Seeschifffahrt und Hydrographie“ and all maps are a licensed copy. They put a lot of work into maintaining and updating those maps and one way to maintain the infrastructure for safely traveling is paying for it.
2. Your life could depend on an accurate map. The official maps I found were updated every 10 days. Would you risk it for a couple of €?