Isn't the porthole thing a popular misconception?
See #7 here:
'7. Parts were inspired by a diving helmet.
Apparently after having received the design brief for what was to become the Royal Oak, Gérald Genta happened upon a diver emerging from Lake Geneva and thought to himself, If the screws holding the faceplate were strong enough to ensure a watertight seal on the helmet, surely they must be capable of doing the same for a watch.'