Does the engine continue to idle normally, or does it sputter and surge? If the first, it sounds like an electrical problem. Most boat's wiring leaves B+ available for the blower, radio, lights, bilge pump, etc. in key 'off'. Make sure you don't have your 'run' circuit tied into these. With the key off, you should have zero voltage between the '+' coil terminal and the loose '+' wire. If there is 12V, you need to recheck your ign. switch wiring.
If it sputters/surges the engine is running on due to several causes.: Hot carbon build up on the exhaust valves or plugs. Mine did this until I tweaked over-rich idle mixture screws more to the lean setting.
