What does the other manifold look like?

looks perfect no discoloration or anything at all what so ever which I find weird lol!

You probably only over heated one side of the engine…which is why you didn’t get high temp alarms.

sensors were happy until kaboom.

Looking at the picture of the manifold the heat centered around the top of the manifold. The paint around the exhaust ports looks of ok which is really strange if you had a cooling (flow) issue. Maybe the V Brand flange cracked and introduced air into the stream causing a lean condition which caused a backfire. Never seen anything like this before and I've seen many fried headers and manifolds from flow problems

We all know it’s from heat. But what else could cause that much heat and not melt other things?

We have had cast manifolds crack/blow holes in them on OEM CAT manifolds when they are miss firing. The unburned fuel hits the HOT CAT section of the manifold and it explodes. Cracking the cast manifolds. The cracks/hole we see look exactly like this.

Wondering if that spot is hot enough to ignite unburned fuel and causing an explosion right there. Making it even hotter and crack/blow up right there.

Just a thought from what I’ve seen look like this on other manifolds.

I don't think the stainless riser that is water jacketed would get hot enough to turn a straw color.

Likely a water flow feed issue to the port exh man.
The exh man had water in the jacket with sufficient flow to pull some heat out and transfer it to the skin of the outer jacket. The low/zero water flow heated up the water fed to the riser, letting the riser heat up to color.

The unjacketed portion of the exh man was unable to conduct away, heating up to a point where mounting forces overcame the structural integrity.

Late timing would affect both sides. Fueling would likely affect both sides.

Was the hose feeding the riser from the exh manifold pinched enough ro limit the flow?

call me crazy but I find it tremendously easier to work on these engines outside of the boat and I can have them out and accessible in less than a hour anymore lol.

so started at the easy part. Impellers. Which where new the season before. They seem perfect. Wear plates look at have some wear but nothing I don’t see being “ not normal “ no debris as of yet blocking anything either. Pulley spins freely and has a spot it grabs for lack of better words which I think that’s when’s engaging the fuel pump? I’ll keep tracing from here.

This is the other manifold on the same engine.

Oil cooler is clean and clear

I wouldn't call that impeller perfect. I see the surface that rides on the wear plate as a potential issue