I am not a lawyer, so take this with a big grain of salt: I believe the issue on using OSM data is making private changes or in radically transforming it to combine it with other data. And I think the way around this is: 1. Have a separate private database with your proprietary data. From your question it seems that the depth data is the only thing you need to keep private. 2. Submit all your road, buildings, shops, pubs, ATM and shoreline data updates to OSM. When you make your actual map(s), pull data from both OSM and your private data. In this case your map is being created with "collective data" per http://wiki.openstreetmap.org/wiki/Legal_FAQ#3d._If_I_use_your_data_together_with_someone_else.27s_data.2C_do_I_have_to_apply_your_license_to_their_data_too.3F as you really don't need to transform the OSM data to merge it with your proprietary data. data. You are likely to simply use the OSM data as a base layer and then add a layer on top of that with your depth information. You will, of course, need to acknowledge the source of the OSM data to use it.

I am not a lawyer, so take this with a big grain of salt: I believe the issue on using OSM data is making private changes or in radically transforming it to combine it with other data. And I think the way around this is: 1. Have a separate private database with your proprietary data. From your question it seems that the depth data is the only thing you need to keep private. 2. Submit all your road, buildings, shops, pubs, ATM and shoreline data updates to OSM. When you make your actual map(s), pull data from both OSM and your private data. In this case your map is being created with "collective data" per http://wiki.openstreetmap.org/wiki/Legal_FAQ#3d._If_I_use_your_data_together_with_someone_else.27s_data.2C_do_I_have_to_apply_your_license_to_their_data_too.3F as you really don't need to transform the OSM data to merge it with your proprietary data. You will, of course, need to acknowledge the source of the OSM data to use it.