Hi there. You don't have to actually land on them, but you do have to dive really close to begin with. If you don't start off right on their tails, you should probably resurface. But when you're moving underwater, you always get a burst of speed before you run out of breath, and that's usually the only time you will catch a spider crab. Good luck!
Apparently, every item is "rare" on the trading board.
I'm a mobster - I deliver flowers. ;)
Quick tip! - As you begin to run out of breath, you begin to swim faster (Already mentioned) - Before even diving, position yourself so that your target doesn't swim towards the swimming boundaries or the beach, otherwise you won't be able to catch them if they make it past the boundary or beach (This doesn't apply for stationary targets). - If for whatever reason you cannot position yourself properly (target to close to shore/boundaries), ignore it because it'll just be a waste of time. - Just practice! The more you dive, the more you become experienced at it! Also you become more coordinated with your location when you dive. - You swim faster on the surface than underwater (Kinda obvious). Use this to close the gaps before diving! When you dive, the gap distance between you and the target should be around the size of your shadow underwater (This applies for fast critters like the crab).
With these tips, you'll breeze through the snow crab tour!
Oh wow I didn't think it was TOO hard... the first time I did that tour I caught 9... But sometimes they were a little hard to catch because they went behind the barriers. Try to get them away from the barriers before you dive, don't chase them towards to barriers because you won't have much time to get them before they get out of your reach. If you try the tour again, then good luck!
EDIT: Oh sorry Teemo I didn't see your advice until after I posted this, I basically said the same thing as one of your hints... haha