Hi Brian,
At least it was just your outboard, and not a person.
Rinse with clear water.
Dry the unit out thoroughly, Pull the plugs and turn the motor over to
clear the cylinders. make sure the carbs are clean and fire it up as
soon as you can. Change any oils including the lower unit.
Your battery is toast. replace it.

Capt Frank
http://www.home.earthlink.net/~aartworks

Brian Klug wrote:
Hi,

Need any help / advice on this subject. My boat, along with my 115HP
Merc outboard was completely submerged as a result of hurricane
Isabelle. It was underwater for about 18 hours, with battery
attached, in brackish water.

It's had time to dry now – should I just try to start it and see what
happens? Or would that likely risk further possible damage? Any
suggestions?

Thanks so much,
Brian Klug