A good friend of mine just replaced his 2006 twin F115 Yams with just 365hrs due to in internal corrosion on both. He did flush them after each use but perhaps the the previous hadn't been so mindful. I work with a Yamaha outboard mechanic skippering his boat at times who says it's pretty common with those older engines. Check the powerhead anodes are kept up to scratch.
By the way he's gone to a single Mercury 225 rather than replacing with twin motors again and the performance difference is night and day, from handling, power and economy.
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