Sea-Dweller and it's depth rating...

I'm going to throw out a couple questions. As I said in a previous post, I'm definitely not the physics guy...so maybe you folks can help me out with this. First of all, before I get to my MAIN question, just as a sidebar, if you have a watch, for example, the Sea-Dweller, which says it's good to 4,000 feet, does it literally mean 4,000 feet beneath the surface? Or is that 4,000 feet of pressure? Or is it the same thing?? I guess what I'm trying to ask is, lets say you're underwater 20 feet below the surface, and you're stationary. Then you start swimming. Once you start, does your motion, because of the resistance of the water, thereby exert MORE pressure on the watch, even though you're still at the same depth of 20 feet? You folks see what I'm getting at? Just kinda wondered exactly how that worked. As far as the main question thought...I was curious...why do we need the new Deep Sea to be rated at almost 13,000 feet? And I ask because, and maybe I'm wrong here, someone please correct me if necessary, but since no one will ever be going down that deep unless you're in a pressurized submersible, then technically, it wouldn't matter what the depth rating is, because the submersible is pressurized, isn't that true? That would mean that, in a pressurized submersible, at 10,000 feet below, a day date is as good as a GMT-Master is as good as a Deep Sea. Or is there something that I am seriously overlooking here?? I'm not here to do any bashing, you folks know that I do love my rolexes, and I appreciate the impressive statistics of the new Deep Sea...I'm just trying to figure out some physics here...any one have any thoughts on this????