NOAA Marine Charts for USA Waters
Forever-free Access to All RNC Charts
NOAA Nautical Charts: Safety While You Sail!Coverage at Multiple Scales
Overview of Massachusetts Bay at 1:378838 scale.
Great detail in Boston Harbor at 1:10000 scale.
There are also charts for this area with in-between scales of 1:80000, 1:40000 and 1:25000.
Downloaded charts switch automatically as you sail.
You have the information you need in iSailGPS.