I thought the aircraft carried by the M class sub was a seaplane?
It was pulled from the deck hangar the sub submerged and the aircraft took off from the sea and was recovered in a similar way. You could be correct of course,I had a picture of the M2 years ago I could be mistaken but I'm sure it had floats.
There was a documentry about the search and location of other M class sub recently, it carried a 12 inch gun on deck.