Usually the shape of the blades is different as they work in different conditions
http://www.icboats.org/boat/document...nformation.pdf Here you have some good information.
Submerged propeller as a rule is more thinner in the trailing edge area.
About the Octra 1240 I think they work well as submerged but as you can see in this site
http://modelmarinesupplies.co.uk/sho...65e894b1708acc
They are too big for your engine.
I would try a mocom 1816 or a plastic Radio-Active X-35 5mm. The Octura X-438 may work (but it seems to me they are better for surface).