Absolutely not overkill! Although nothing can replace paper charts for reliability (They are waterproof and don't depend on your boat's electrical system

), a GPS/Chartplotter exponentially increases your margin of safety when operating a boat in any waters. A depthsounder of some type (whether a numeric readout or "fishfinder") is also a virtual requirement to help ensure you aren't operating in waters too shallow for your boat.
As for the type, well, it's all about what you use your boat for and how much money you want to spend. The sky is literally the limit in terms of capability. I'm not real familiar with the Maine coast, but if fog tends to pop up suddenly like it can down here, a radar and/or AIS system may also be a good idea.
This isn't a solicitation by any means, but I am a Raymarine Certified installer and Service Tech, Have been a factory trainer for both Lowrance/Eagle and Garmin, and am an authorised reseller for Navico brands (Lowrance, Eagle, Simrad, B&G, Northstar). If you would like to discuss the options available to you, please do not hesitate to call me or send me an email. I will be more than happy to work with you to find the best solution for your boating needs and direct you to the appropriate sales outlet in your area based on that solution.
My number is 757-403-2134 and my email is
[email protected].