Suggestions for Open Water Training in Cape Cod

I'll be spending a week with my family in Eastham, MA on Cape Cod later this month. The last time we were in the cape, we stayed on a lake and I swam or extended periods every morning while everyone else was having breakfast. This time we couldn't find the right accommodations on the water. Obviously there is lots of water nearby and we will be close to both the ocean and the bay. I would be grateful for any suggestions about where to pursue some early morning open water training before starting the family day. Thanks!