The Internet being what it is, someone got curious about this issue and ran the actual numbers. As it turns out, movement does indeed increase actual pressure, but to a trivial degree. The relevant forum post on Watchuseek includes this remark: "Without repeating all the calculations here (they involve denominators and the Greek alphabet and are PITA to type out), at a depth of 330ft(100 m) and moving your arm at 3 ft/sec, the dynamic pressure is in the order of magnitude of 0.14 feet of head or 0.04% of the depth. Even assuming you could move your arm at 20 ft/sec (14 mph!) the dynamic pressure is only about 6.2 feet of additional depth (<2%)."