Can the vintage Seamasters with normal non-screw in crowns really be that suitable for any sort of underwater activity even when purchased new?

 

The screw down crown is not really an integral part of a watch being water resistant. There are plenty of water resistant watches out there that do not have screw down crowns. The screw down crown is more for ensuring the crown is not accidentally pulled out while under water.

There are 2 considerations when determining if a specific watch is water resistant...design and condition.

Some watches are just not designed to be water resistant, so they may have features like dust roof crowns, snap on case backs with no gasket, etc. Even if a watch has been designed to have water resistance, of course the seals and sealing surfaces must be in good enough condition to do their jobs.

Cheers, Al

 

Thank you Al,
I always assumed the screw in crown was to guarantee water resistance.

 

The seals are what create the water resistance. Some watches have a seal so that when you screw the crown down that seal mates to the face of the case tube, but that is not the only or primary seal.

Cheers, Al