Since I'm a sailor (and an embedded systems programmer) I have been using windy for several years now.
I also use passageweather.
Passageweather has a great feature that allows to download weather charts (wind, waves, etc) to the laptop in zip format.
This allows while sailing away fro the internet to consult the weather forecast obtained on shore.
Is this feature available on windy? Will it ever be? thanks

Regarding free world maps try here:
map.openseamap.org
www.openstreetmap.org

The problem with depths (seems that openseamap organization has having the same problem) is that they aren't public. Openseamap had a project going where they would ask sailors to have depth sounders with GPS on board yachts to collect data.