carbed engine sans t-stat?

i've been wrong before, but I thought the thermostat was located under the coupler that joins the 3 hoses and feeds the hose to the front of the engine. its not there. so i checked down inside the coupler, toward the distributor. nothing there either. not sure where else to look..

351W so EFI temp is not an issue. and no heater in the boat. I thought i read something about fuel efficiency being off when thermo isn't working properly.

Temp guage was going on the blink end of last season. I checked the impeller again and just decided not to worry about. I want to fix it before we start riding a bunch. Is my temp guage itself going bad? and do i need a t-stat?

I had a merc 350 with a carb so no EFI, but i specifically remember when I had a bad thermostat (stayed open all the time) that the boat shop told me it was important to let the engine heat up before cold water entered the block. It allows everything to expand and seat properly. May be different in your case cause of the EFI but i doubt it.