Foundation next to salt water, on bedrock
What I have done in a similar situation (not bedrock, but a huge boulder) is to make (rather tediously) form boards that closely fit the rock, and drill holes into the rock to insert steel reinforcing bars into, which are tied into the slab reinforcing steel. Similar holes would go into the existing foundation. 

Neither may strictly be needed - bedrock is about as firm a foundation as you will get, so there should be no relative motion, and no tie-in whatever may be just fine. But it might be reassuring to at least have the bolts anchoring the walls drilled and grouted well into the bedrock, when the wind gets howling....

A local concrete company would be a better source of information about details relating to the satwater exposure and any specific details related to that (hasn't been one I've had to deal with, but I vaguely recall that there are things one does to seaside concrete to increase the longevity in that hostile-due-to-salt environment.) Presumably at least one part would be to wash down the area where the new slab will go with fresh water to remove any salts, before pouring concrete.