I sugest to add a new profile "sea" for navigation on sea (near coasts). It woud be simple if you take the direct line without islands and barricades. OSM is quite suitable for this useage. The positions have to be taken without asignment to a street on land.
This might be a nice addition but simply an extra profile would not suffice.
You need to know where you are allowed to sail, where the fairways are, where the buoys are, where the (dangerous) currents are, where the sandbanks are, etc.
I know from which app you got your idea, but that one does have a lot of data which is necessary to do the above.
And of course the "stick to road" in the navigation should be switched off for that specific profile.
However, you could simply create a simple gps track and follow that one.