Buy replacement keys for your boat from the dealer, a marine store or even Amazon.com. Boat-motor manufacturers such as Honda also offer replacement keys online.
One of the fastest ways to acquire new keys for a boat is to contact the original dealer that sold the boat or motor while having the original purchase information handy. This helps determine the exact key type, as each manufacturer offers more than one style of key. Otherwise, contact a nearby marine shop that carries the same brand of boat motor; the dealer may have several key types in stock. A locksmith may also be able to make a replacement key for the boat.