If you need coastal shapes cut out of the ocean, try using the Pathfinder tool. Make a rectangle to represent your ocean, place all of the landforms on top of it, and select everything. Then use the Pathfinder to subtract the landforms from the rectangle. You may want to make a backup of the landforms first because if I recall correctly, the straight subtract feature deletes the objects used for the cut-out. It's been a while since I last used Illustrator, so I don't remember the exact details, but if you experience with the different modes there you should find one that will do what you want.

Of course, you'll need to be careful thereafter to never nudge anything's position or your ocean will lose registration with the land.