Yah, that's probably the best bet albeit farther than 15 miles from shore. Dead bodies float, zombies are undead, so I'm pretty sure they could swim/float 15 miles out. Make it 100 miles and probably should steal a fishing boat. Could use the lines or nets to make a protective barrier around the boat while you're waiting or catch fish for food.
Also, make sure to bring buckets to capture rain or make drinkable water from sea water (clear plastic + the sun = drinkable evaporated water).