I'm with Ian (itsonlymoney) on this one: I'll bet you anything you like it's the oil level in the outdrive. This tends to "shake down" when the oil is changed as part of a leg service, and usually needs topping up at least once. Assuming that you have a Mercruiser of some kind fitted to your boat, the reservoir for the leg oil will be to port somewhere on the engine and there will be two bands on it, one for full, and one for top up level. If it's low, top it up and I bet the alarm will, magically, cease. If it's not a Merc I can't help I'm afraid