Welcome to socaljetboats

98% of all jet boats DO NOT run a t-stat.
This is a single pass system. you want to gain about 50*.
Lake temp 75*, outlet temp 125*. DO NOT run 180-195. ITS NOT A CAR.
McMerc's run 'tween 140-160, with a stat. Outboards usually cooler, too.
Oil temp is a much bigger factor.
use a gate valve, at the pump, to get some temp out of the motor.
good luck,
Dan'l