Your best option in my opinion is likely to just buy a new OEM ignition key switch. A new OEM switch should be a simple replacement--it should either plug-in or wire-in with no modifications. I doubt that there exists any aftermarket outboard engine ignition key switch which would have particularly better tolerance of surviving heavy splashing or spraying with sea water.
Key operated switches are, in general, difficult to create with 100-percent-waterproof characteristics because they must contain some sort of slot for the key to be placed in the lock. The only way I can see to achieve 100-percent-waterproof characteristics would be to use a conventional mechanical lock and key mechanism, then link the action of the key and lock to an electrical switch that was hidden behind the panel.
The better outboard engine ignition key switches usually have the actual key mounted in a rubber cap assembly. When the key is inserted into the lock portion of the ignition key switch, the cap assembly covers the opening of the key slot. Check to see if your particular ignition key and switch are available with the key mounted in such a cap or cover assembly. Here is the ignition key for my c.1992 Evinrude engine, nicely mounted in a rubber shroud.
- OMC_IgnitionKeyWithShroud.jpg (12.77 KiB) Viewed 8545 times
OMC ignition key mounted in rubber shroud to prevent ingress of water into ignition key switch slotOMC figured this out a long time ago. When their ignition key is installed in this rubber shroud, it is much harder for water to get into the key slot of the ignition key switch. Also, there is a specific orientation for the ignition key switch because there is a drain designed into it to allow any water that does get in there to drain out. The switch has to be properly oriented to allow the water to drain. If you put the switch in upside down, you cancel the draining action.
You can also significantly improved the protection of the OEM switch against intrusion of seawater by changing its location or by adding a splash guard to the switch in its present location.
Another option is to abandon the OEM-style ignition key switch and create your own control system using several separate switch to perform all the function of the OEM ignition key switch. I don't really recommend this unless you were certain that the boat would continually be used in an environment where the key switch would be subject to continual exposure to seawater spray and soaking.