The motion of a hand through the water is nothing compared to the static head pressure of the water.
The pressure from the water is basically 0.43 psi for every foot of depth. I know some physics, I have a masters degree in engineering.
Did you specialize in fluid dynamics?
So, 1 meter=3.3 feet, that's 1.4 psi. That's all the Watch is rated for -- in a static tank with no external forces just the column of water above it. So now strap that onto the arm of a swimmer, which will not likely submerge below 1 meter during a swimming stroke. So you're asserting that at no time is the psi on the watch greater than 1.4 during a typical swimming stroke?
I couldn't easily find any specs on the exact amount of force against a swimmers hand as it strikes the water, though there have been many studies that have measured the pressure of the applied forces on the hand as it rotates through stroke. However, I did find some info on the psi of various kinds of boxing punches. A basic martial arts punch appears to create about 178 psi on average, all the way up to 1000+. So, I would imagine that the force of the hand impacting with the water is going to be at least somewhere between 1.4 psi and 178 psi. And I'll wager at a minimum considerably more than that subjected to an object 1 meter under a column of static water.
While dynamic pressure doesn't really apply over a certain depth, since the force a swimmer is able to generate with their arms will eventually be negated by the pressure of the depth, there is no doubt that a swimmer is forcing water against the Watch far greater than that for which it's currently rated. And while it seems likely the Watch can withstand far more than that ipx7 rating, the real question is how much more given it's not just sitting at the bottom of a tank, but actively, and repeatedly, being impacted against the surface of the water, and forced through it with enough energy to propel a swimmer forward on the order of 2m/s. Even at a 100 meters under water the pressure is only about 140psi, so depending on the force of of a swimmers hand impacting the water, and assuming it's somewhere in this range, only then does the rating start to protect against swimming. To say nothing of swimming in the ocean where the forces of a moving column of water can generate even more significant surface pressure in the form of wave action.
So far all we have is anecdotal evidence of how well the Watch survives in the water. The "tests" such as they are have by no means been comprehensive. In other words, following depth pressure tests, the watches have not been opened to examine for water ingress to know how effectively they "passed" these tests. And I'm not sure that any have been subjected to high pressure hoses at all and other common tests which could account for some common forces in water sports activities like, swimming, high diving, water skiing, jet skiing, surfing, etc., much less inspected afterwards. So I don't know how anyone can say all Apple watches will survive the impact of sport swimming on a regular basis in every situation a swimmer might encounter, with a certification of only 1.4psi in a static tank, in the absence of proper forensic testing.