I will look up the catalogue to see if the association has drawings for this exhaust – but I dont remember seeing such a drawing.
While I cannot claim expertese regarding this specific engine installation, or even to have seen an engine installation in an A31, I can claim experience of similar systems.
It sounds like your exhaust manifold is too close to the waterline, or perhaps even lower than the waterline. A fairly standard problem with standard solutions. In modern installations this requires a syphon breaker in the water inlet line between the sea cock and the water pump. It also requires a “swan neck” in the exhaust to raise the pipe up to deck level before it falls back down to the exhaust outlet so that water cannot flow back down the exhaust. This modern practice would use a rubber exhaust pipe and inject the water into the exhaust just where the exhaust becomes rubber. A low section in the rubber pipe is used to allow the water left in the exhaust to settle into. The two elements go together. The swan neck stops the water flowing in from the exhaust, and the syphon breaker prevents water from flowing in through the water pump from the cooling water inlet.
Now to your installation. Since you are getting water into the engine I suspect that your engine is low enough to need the precautions mentioned above.
You have not mentioned whether you have a syphon breaker in the inlet line. You do need it.
If I describe the exhaust fitted to the A26 when new you can let me know if yours fits this description. The exhaust from the engine to the transom consisted of two pipes one inside another. The water ran between the two pipes so thet the inner pipe was a dry exhaust, surrounded by a water jacket. Near the transom there was a box where the water was mixed with the exhaust just before it went overboard. This box acted as a swan neck.
If that sounds like your exhaust then it probably has corroded through internally and is allowing water back into the engine. It will not be repairable if it is this old, and having a replacement made as a copy will be expensive. The solution will be a modern rubber system with plastic silencer, swanneck and skin fitting. I suspect you will have to make an injection bend as well.
I think that you need to replace the whole exhaust anyway if what you have is the original. You should not take chances with exhausts because of the risk of poisonous fumes entering the accomodation, and the risk of engine failure.
Hope this helps