Question on Raw Water Cooling Routing with Dry Exhaust ?
 

I've got a 1987 36' Mirage Twin 502's no thermostats = No Temp, If I install T-stats the Exhaust wont get wet until the T-stats Open . NOW WHAT ???

drill four 1/4" holes in the t-stats as a bypass until the t-stat opens.

The stock T-stat housings along with the pump should be in by-pass for the exhaust always. I would put a T-stat in and check the exhaust hoses to see if water is going through them or not.
It will either come out the tail pipes or not. If not, you have a plumbing problem.

Here is what it should look like on the tail pipes anyways.
http://www.youtube.com/watch?v=Q1QC8VYxoTQ

I agree with jvthundercat. You should always have a bypass in the t-stat housing or the cooling plumbing.

and if no by pass . you risk a water pressure problem at higher speeds plus the heating of the headers issue . want to keep them headers cool... eddie marine make a bypass complete kit i might have two of them lying around here i can show you a pic . i just runs a line from the intake water ports in front of motor to the top of the t stat housing . always floiwing water to the manifolds . it sill diddnt bleed off enough water pressure at high speeds . needed to add blow off valves like mercury does .. just saying . good luck