1 - do you have a blackberry data plan? (I don't know about google maps on just carrier data. Someone else can answer that.)
2 - If you have access to mobile data so that you are able to download the maps, and your blackberry can "see" the gps satellites, you will see your position on the map. Being on the water has nothing to do with it. Having a clear view of the sky and data service are the keys.