Boat is 700 miles away and no locksmith near boat and no key to match it to
What brand is engine panel (was it for a Volvo? Yanmar? Westerbeke?). Panel part number?
If there's a number stamped on the front of the key switch, or a serial number on the panel or key switch, there's a remote chance that the panel manufacturer, or the maker of the key switch itself would have some sort of registry for what key is required. The info might be in the original boat docs, assuming you still have them. I have no experience to back this up, but this sort of thing is done elsewhere (eg cars)
Knowing the panel's make and model number, someone would no doubt be thrilled to sell you a new ignition switch ($$$).
A good boat mechanic or electrician would probably be able to trace out the wires and replace the key switch with a 'universal' sort of replacement key switch, or a toggle and a push-button if you want to get away from the key.