Not likely. First off, with an antiquated propeller driven pontoon plane such as this, you're going to need a heck of a lot more than 30 ft. to reach 50 MPH.......
To launch the plane, the sub released the holdings on the pontoons, went under water then let the plane do what pontoon aircraft usually do while on water.......take off.
The 30 ft. deck was designed to accomodate the sub's ability to surface under the pontoon plane once it had landed on water.........
Could be but does that mean the 3rd photo is of the plane doing a close flyby rather than taking off?