I have painted my Lapworth 36 in the water and on the land. I would not do it in the water again as it was way too hard on my back. I just completed painting half way down the hull and that is most of what you see when you walk by, isn't as hard on the back, and came out nice. I would not suggest you do it that way, but I had cracks to repair as I do every year and you can't haul out every year and you can't paint the entire hull every year.
The thing that might be useful is that I bought a new vacuum from Home Depot, a portable 4gal 5 amp. I used a long hose I got with another sander and a 5 inch Bosh palm sander. No sanding dust at all in the water. By comparison, sanding a 1/2 inch by 1/2 inch spot for 3 seconds without a vacuum puts a lot of dust in the water.
This time I used a brush but previously I used a roller and a brush. I would never use a roller and brush again even out of the water. I think that is the way to go if you have two people but it is just easier to stick with a brush than switch back and fourth. I used a 2 inch brush, a good brush very expensive, but not that big. It was easy but I was not going all the way to the bootstripe.
When in the past I painted to the boot stripe in the water, I swam around the boat putting the tape down. I certainly would not attempt doing the boot stripe in the water as you will kill yourself getting the sander in the water. If you insist on painting in the water, at least do the boot stripe when you haul out. You might also consider doing the sanding and prep in the water and doing the paint after you haul out.
One final thing to consider. If you paint out of the water you are looking up at the hull. Any sag or drip looks terrible from that angle. It looks much better from the dock and much better standing back a few feet than when your face is against it on the step ladder.