Yeah speed breaks masts in DW. The depth thing is obviously not realistic but its a tiny detail. In any case beyond a certain depth you can't see anything through the periscope or the photonic mast. Just below the surface of the water instead you will be able to see the hull of ships, and submarines going overhead. This makes for very exciting "spy" missions. Yeah I know that high frequency will let you see, but visually seeing through the periscope is much more fun.
