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I agree with Phil. Without knowing why a relatively good battery was dead, adding a second battery is just adding complication. What loads are present when the boat is not being used? Any relatively cheap voltmeter can measure small current values, and are a really valuable tool in a boater's bag of tricks. Hook one up inline with the battery when you think everything is off and see if there is any current draw.

I just reported my experience with a faulty bilge pump in another thread, and that led to a dead battery on 2 occasions for me (once with a brand new one).

I do carry a booster pack in my car (and in the boat for any significant outings). They are much cheaper and less complicated than adding a second battery if it's just a dead starter that concerns you. If your house loads are much more significant, then a house battery should be considered.

Dennis