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I think even more important than the sonar unit, is actually studying charts before entering an area. The depthfinder should be accurate and its really easy to go from very deep to taking off your lower unit in seconds.
Boston Harbor, especially the outer islands are all about ledges and its not usually going to be a gradual slope. That's what makes them good for fishing. Depth finders are great, but by the time you see the depth change, you may be way too late.
Study the charts and use your sonar to aid you.
And don't rely on sight either, cause alot of factors could lead to not being able to see a boulder just under the water. Glare, murky water, chop, cloud cover, etc. Polarized glasses help, but not always.
I study the charts before going out, and again when I get back in, to help myself remember mainly the danger spots... where to stay out of.
I also have a chart of the harbor on board in case I find myself in an area I don't know (or wanting to get into an area I don't know).
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