Diagnose my spark plugs...blown headgasket?

[nuron]

engine: Mercruiser 5.0 305cid

Took the boat out last weekend and it shut off after cruising, and I think I have blown HG. Took out the plugs today and this is what I found (see attached). Four questions for you guys:

1) has #7 seen water, seeing that green corrosion?
2) why is #4 so clean? that one looks like it's seen more fuel than the others

If my HG is indeed blown:

3) Is there any chance of a cracked block from this situation? I'll be doing a compression just to see but I know leakdown test is preferred
4) Any advice on a HG replacement? Is it worth the effort or should I just swing for a new block? I've read somewhere people used a straight edge and feeler gauge to check for head distortion and if it's still flat, just replaced the HG without machining the head.

Thank you in advance for any advice or tips!

 

[jimmbo]

And why did you suspect a blown head gasket? Was the engine running rough?

First thing to do is a Compression test.
What you call #4, which is really #7 in a SBC, looks like it never was exposed to fuel, and is like brand new. The others look wet

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As for the one you call 7, which is really #6. hmm. How old are these plugs?

 

[Bondo]

Ayuh,..... Welcome Aboard,...... The plug you have marked #4 has been steam cleaned by water intrusion,....

The plug marked #7 appears to be corroded,....