I know this is an old post, but it's worth posting for others. What Gazgen is describin sounds like 'Reversion', which can dump water back into the engine through the exhaust valve's, even when the manifolds and risers appear structurally sound. The actual term is 'Resonance Reversion'.
here is a good doc which describes this process:
I've seen this happen with aftermarket manifolds which internally aren't designed quite the same internally as the OEM parts, such as Genuine Mercruser parts. There should be a small internal lip which should prevent reversion from allowing water to backlow into the exhaust manifolds. I had two friends who experienced this shortly after replacing Mercruiser Mainfolds and Risers with Osco manifolds and risers because (supposedly) the Osco manifolds and risers didn't have this lip or the lip isn't placed and /or sized correctly to completely prevent revesion. I've heard tell of a situation where the riser and elbow were not sized tall enough to prevent reversion as well. the doc above discusses this in detail and is worth reading by anyone who uses exhaust manifolds and risers.
This is just one man's opinion (FWIW) so for those who use and swear by aftermarket or Osco specifically may have a different opinion.