I'm not familiar with that outboard, but agree that it would be nuts to not have a separate thermostat. There are several good Mercury parts sites, I use BAM Mercruiser Parts for lookups. Don't always buy from them, but they have a good look up. Plug the engine serial number in the top right of the home page then navigate down (probably) through 'raw water cooling' and you will get an exploded parts diagram and table of parts.
Hope that helps.
