Im running an 18 foot 78 charger bubble deck jet, stock Berkley with 502 with AFR heads. I recently had it in the machine shop because my motor ate the windage tray, machinist was as blown away as I was with the situation and how it happened?? But I got the motor back from machine shop and ended up having to take it back due to some machining issues with the heads, upon inspection, on top of the machining error, my machinist said I was about to blow a head gasket (we had put maybe 5 miles on the motor at this point.) Machinist said possibly due to detonation. So pulled timing out, changed to a different spark plug (running the recommended plug for the AFR head I have). I run 91 pump gas unsure of my compression ratio cam lift is 635 intake 640 exhaust 112 lobe separation. But I was out for memorial and blew the head gasket, my water temp is running about 150 degrees, I have water flow coming out of the pisser on the side of the boat and I walk my boat into the shoreline to ensure there is not any sand going into the engine. I have 20 miles on this motor now and the passenger AFR aluminum head is toast now. Any input would be greatly appreciated.

Sorry to hear about your troubles.

How did the old spark plugs look?   You would see aluminum specks on the porcelain from detonation.

I'm sure that the machine shop checked the flatness of both the block and the heads.  Just to be sure, what were the findings?

Different types of gaskets require different surface roughness, achieved during the machining process, in order to seal properly.  Is yours appropriate for the gaskets you are using?

I have to ask, because I see a lot of improperly plumbed boats.

Do you have the water line, from the suction housing, plumbed into the front of the engine block, at both water pump ports?  There should be a tee or wye so that each side of the engine block is supplied with equal water flow.  The water jackets on each bank are separate and there is no flow from left to right.

You didn't mention what type of intake manifold you have.  Most use some type of automotive manifold with a thermostat provision.  The water should exit the timing cover end of the heads.  Through the thermostat provision, cast into the manifold, is a good choice.  If you have a manifold that doesn't have a single outlet from both heads, you need an outlet hose coming from each head, to a tee or better yet, a wye, and eventually dumping overboard.

There are various ways to plumb log exhaust manifolds, either before the engine, to preheat the water or after the engine.  I don't want to get into a debate over log manifolds, I'm only concerned that both sides of the engine have sufficient water flow.  You did say, you "have water flow coming out of the pisser on the side of the boat," that would indicate dry headers.  How much water flow?  Hopefully more than a trickle.

The following is my opinion, and it is not a popular opinion:

150-degrees is too hot for your water temperature.  Water temperature really means nothing.  What is important is the temperature of the components of the engine and how they relate to each other with clearance to move properly.  Oil temperature is a better indication of what is happening inside your engine.

The cooling water is plumbed off the suction housing, between the trailing edges of the impeller and the wear ring.  If you have an A cut impeller, there is 1/8" gap around the outside of the impeller, between the impeller and bowl, which is a cross-sectional area of 1.78sq/in.  If you have a AA, that gap reduces to just 1/32" with an area of 0.45sq/in.  Both areas are much bigger than hoses and fittings downstream.  However, the diverter nozzle is a huge leak and I don't think the engine gets sufficient flow during extended periods of ideling, such as ideling through the channel at Havasu.

Every hose, fitting, and the water jackets within the engine itself are a restriction to flow.  If a smaller impeller is installed, the water flow through the already restrictive system is reduced at any given RPM.  Example, water flow at 3000 RPM will be greater with an A cut impeller than with a BC cut impeller.

What does all that mean?

Flow is going to take the path of least resistance.  The diverter nozzle is much bigger and a shorter path than any part of the cooling system.  At idle, you will have much less cooling flow than when under way.  The water is going to get hot at idle

While under way, water flowing through the cooling system will have a higher velocity compared to that of a car.  There will be less time that the water is in contact with the surfaces of the water jackets.  To see an explanation of this, see https://sciencing.com/calculate-time-heat-water-8028611.html

If the velocity of the water is slowed so that the water temperature reaches 150-degrees, the water is moving too slow to properly extract heat from the engine.  That's more than doubling the ambient temperature of the lake water.

To operate a car engine, with a closed-loop cooling system, at 160-degrees, the temperature of the water coming out of radiator is not half the temperature of the water entering the radiator.  The (cooled) water coming out of the radiator is way above ambient air temperature, enough to burn you if you grab the lower radiator hose.  The number that I remember is that a properly functioning automotive cooling system should keep the coolant temperature about 80-degrees above ambient air temperature.

In a jet boat, while under way, you should be able  to touch the front of the head without blustering your finger.  The front surface of the head should feel "uncomfortably" warm.  If the head surface is so hot that it will blister your finger, how much hotter are the cylinder walls and valve guides?  There will also be a greater temperature differentials across the decks etc., increasing distortion.  Keep in mind that you have ice-cold lake water being used to cool the engine, not water that is already hot enough to burn you, as would be coming out of a car radiator.

I'm also not a fan of thermostats on jet boats, unless you have a closed-loop cooling system.