As your posting in the saltwater section then I guess your a coastal rider? If so its more likely a corrosion issue.
The elbow or riser will have corroded to the point that the metal around water jaket is so thin it has cracked and allowed water into the engine.
If you spotted this early enough you might just get away with replacing the manifold and riser. If not and you've allowed water to sit in the engine then you could be in for a rebore, new rings, shells and bearings.
This happen to 2 club members last year on thier 3L mercs. One left it a little too long and is now running in his brand new engine.
If you flush after every use you should get 7 - 8 years from you manifolds but its adviseable to remove them after 3 - 4 years and have a good check inside to prevent this from happening.
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Keeping it real for the east coast massive.
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