Watching a belly flop in slow motion doesn’t look any less painful than it might be in full speed. The only difference is that the belly flop from only five feet up looks insanely less painful than the one that comes from ten feet up. The Slow Mo Guys are awesome when it comes to this kind of thing and they definitely seem to have a lot of fun in the doing. This was pretty cool to watch though when you really look at how much of an air pocket is being created when he jumps in. Now doing a belly flop isn’t likely the best way to go down, but look at how much more an air pocket he creates when he dives in from a higher vantage point. That’s pretty impressive really but if you think about it from the standpoint of diving then you can see why it would hurt so much.
When you belly flop into the water you’re using as much surface area of your body as you possibly can, which means that everything that connects is going to do so with a horrendous slap of flesh on water. And at five feet up the force generated by a falling body is going to be hard enough, but when you go ten feet up the force is going to increase exponentially in accordance with the distance. Also, the surface tension of the water has to be broken for a body to even enter, so when you hit with your entire body, your ENTIRE BODY has to force its way into the water, and as you can easily surmise, forcing your body through anything is a painful procedure.
Now if you were to actually dive into the water using a swan dive then the point of entry is much smaller and clears the way for your body to enter the water smoothly and with far less friction. Cannonballs are much the same but are typically a little more painful since the entry point is being achieved with a larger surface area, usually one’s backside, which is typically more heavily padded and able to take the brunt of the impact with minimal pain and/or bruising. Imagine throwing your body at a wall, if you stop yourself with your palms the pain will be there but it will be less since your stopping point is smaller and won’t affect as much of your body as if you simply launched your entire body at the wall without your hands up to break the impact.
Doing a belly flop into the water is a lot different than hitting a wall of course, but the theory is the same. The smaller the entry point, the more likely you’re going to feel less of the impact. If you’re attempting to use your whole body to create an entry point though, it will work but you’ll be feeling that impact for the next hour or two, if not for the entire day afterwards.