it is the monsoon 410.

Totally agree with @shawndoggy There are a couple really good salt mitigation products that are available for the engine. YouTube has some good videos on how they work. Plenty of fresh water in the interior. Good luck.

Those look like Australian made boats FWIW.

The coolant used is PG which is environmentally friendly. Indmar, sierra, and starbrite makes it. I just bought a gallon on starbrite on Amazon to top mine off. I must have cross threaded the cap the last time I checked it back it leaked out of the cap. Starbrite is not green but I couldn’t find the Sierra anywhere. I was told the color wouldn’t matter to mix since they were both PG. Here is the snip from the manual and the link for sierra. Was also told to fill just below the seam between the upper and lower half of the reservoir when the engine is cold. 

STAR BRITE Star-Cool Premium Synthetic PG Engine Coolant - 1 GAL (033200) https://smile.amazon.com/dp/B00BNPI06C/ref=cm_sw_r_cp_api_i_64DE5EA1JQ7FF324XYQ4?_encoding=UTF8&psc=1

If the reservoir is empty you should pop the cap and see just how low the exchanger is.  I'm guessing it will need coolant since it overheated.  

Probably should read your manual to see what to use.