Pursi is a free Finnish marine charting app. The app also includes real-time weather, temperature, algae conditions and large vessel traffic via AIS data. The app is open source.
The app is based on excellent open data published by Finnish authorities and the global OpenSeaMap project.
Two key concerns for boaters are the weather and the blue-green algae situation. You can view both directly on the map, without leaving your navigation. You can also report your own blue-green algae observations directly from the app! The sea water situation, wave and sea water height are also included - all viewable from the same app.
Pursi is open source. The app