I've used Weather4D extensively for offshore
sailing. The download process can be lenghty when the navigation zone is large to begin with and as you point out, weather conditions may force to land somewhere unplanned. So it's a good idea to download all the ports
and moorings area you might end up going to if you don't have 4G coverage
Here's how I used the feature so far.
1. I define the largest possible navigation zone and I download the corresponding chart with "Offshore" detail level.
2. I define the coasts I may sail close by and I download the corresponding charts with "Coastal" detail level.
3. I make a list of harbors where I might end up going and I download the corresponding charts with "Ports and Moorings" detail level.
The "Ports and Moorings" detail level downloads are the most precise and also the biggest in size and hence will take longer to download.
The process is tedious and time consuming. There is no question about that. Now this is going to change for the better. GeoGarage and Weather4D are working on improving the algorithm which will potentially increase the download speed by at least x5. It will be also possible to have the download happen as a background task.
When will these improvements be released? They're expected to be available in Q4.