To pick this up again, I called our local timber merchant and he confirmed that you really need to seal any kind of ply once you cut it. Marine will last far longer but always wise to seal the ends.
Now, to my surprise, he recommended that I build houses out of structural OSB. He said the glue was waterproof and it took painting very well. I know they use it to temporarily board windows and stuff but as far as I was concerned, it's not really an exterior product. The offcuts I have used to shelter broody coops etc. have always soaked up water fairly rapidly. It's also not as strong and you can snap it far easier than the same thickness of ply.
So I'm more confused now. Planning on building 6 houses so want to get it right and have something which will last a good few years without spending too much.