A few years ago a friend of mine had a boat with an IO the engine would fill up with water while sitting at the dock. We took it apart several times checked every thing we could. Finely a shop said it could be in your exhaust headers. We tried testing them several ways but just couldn't get it right. The owner made a deal with a shop that had a used set on the shelf. We put them on and all was good. Just something to try cheaper than a block but not by much