You're probably fine then.
I had a similar problem with my former OMG 460 King Kobra.
I had riser gasket leaks in both sides and immediately after shutdown, like when picking up a downed skier etc, if I tried to restart the engine, I would experience a brief hydrolock....If I tried to crank it a couple of times, it would act like the starter was failing/dragging or the battery was low etc...........
If I waited for more than about 30 min, the engine would crank right up! (presumably, after 30 min or so, the little bit of water would leak right by the rings and drop down into the crankcase........since the engine was hot and I'd run it afterwards, that little bit of water didn't last long in the oil)
When I pulled the risers, I noticed evidence of water in the both sides and when I pulled the heads, there was light corrosion in a couple of cyls on both sides.
I was going to replace both the manifolds and risers, so I tried a little exercise with a belt sander. I was able to "clean-up" both riser and manifold surfaces to the point where I had bright shiny metal for the gaskets....and they checked out pretty flat.....
If you try that, you MUST be careful not to take too much metal off and you need to maintain the flatness of the surfaces. If you're not sure you can do it, find someone with a vertical milling machine and have them straighten them up. (although if you have to pay a machine shop to do it, it might be better (and cheaper in the long run) to just replace them
Unfortunately, there were no OEM manifolds to be had for the 460. AFAIK, no one makes them nowadays either. (there's an adapter to put 454 manifolds on a 460 but I know no one that has done that...... they weren't available when I had the 460 in my boat and I now have the 454/BravoIII)
Just do a little block sanding of the surfaces to clean them up, check them with a straight-edge and replace your gaskets........as long as they're not cracked, you'll be fine!
Cheers,
Rick
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