normally,

the lower the nitro%, the hotter the plug.
higher the nitro%, the colder the plug.

if your running too cold, too much cooling or too much oil (needle is too rich).
if your running too hot, not enough cooling or needle set too lean.

I do the same thing on my engines, it is better to restrict water a bit than to go too lean on the needle.

I don't have one boat engine running above 165, only my buggy engine runs at 220.

Dan.