[Mr Schmitt]

I ruled the BMW801 out due to the piston sleeve looking very different to the one in your photo to a BMW801 Engine. The BMW801 Piston sleeve is different as far as I can see to yours (due to all the crustaceans attached to it), as the lower 2/3 of the outer sleeve of a BMW 801 has steel cooling fins on the outside as part of the sleeve and a flange to the lower edge with a circular ring of holes, for the piston sleeve to be bolted to the main engine block and this is a very thick flange. I can now confirm though that one of the valves "is" similar as in your photo. The BMW 801 has two different types of valve for the inlet and outlet. One is concave and one is convex in shape at the end facing the piston top. Each of the valve stems is the same though.

I am surprised that the steel sleeve isn't in better condition, as BMW801 cylinder heads have an aluminium top casting with cooling fins cast into them and in seawater aluminium acts as a sacrificial anode and helps preserve steel. Also if this has corroded away the next item to corrode away would be the aluminium cylinder head. Being a diver I guess you are familiar with electrolytic corrosion. I realise 70/80 years in seawater is tough on steel and aluminium, even after 70 years engines and propeller bosses are still being pulled out of the English channel, very holey, but still recognisable and retaining aluminium props and blocks/heads.

If it is possible to clean the crustaceans off the piston sleeve, then this may show parts of possibly rotted cooling fins and or maybe parts or some of the large flange at lower edge.
If so then we can say it is very likely a BMW801.