When you say it won't start, does the engine go er-er-er or just click click click?
Does the engine turn over slowly or not at all? Are all of the contacts between the battery and engine clean and tight? Are the wires between the battery and engine in good shape? Sometimes the copper corrodes inside the insulation to a point where there is a large voltage drop across the wire, ie, the juice doesn't flow.
Before you send the battery to the great recycling center in the sky, take it off the boat, check the water level in the cells and if needed add DISTILLED water to the battery then charge it.