WWW
I've spent a fair amount of time in an Osprey, like the boat, and think I've always spoken fairly and positively about it. I speced the Merlin II to DY, so know it pretty well, and have hundreds of hours in it, although I consider it an intermediate level hull, neither fish nor fowl. Shearwater I've less time in. That said:
Merlin II may have more rocker on paper but it has less in fact. DY considers rocker a drafting convention, every designer measures rocker in his/her own way and ORC seems to pull numbers out of the air. That explains why the Swifts do better against the current, increased bow rocker gives you better control of the bow.
I tested Bell's YS Solo, Swifts' Osprey and Wenonah's Argosy at the WPASCR last year. All three 30" wide with river rocker in the bow but skegged stern to aid tracking; Same test. Accelerate, hit 2 X forwards, paddle inside circle. Accelerate, paddle cross inside Circle. Do a minimum 180 dg Axle, 180 degree Cross Axle, both heeled towards the turn, 180 degree Post, heeled away. All three did about the same, The Wenonah less stable heeled to the rail the Bell most stable heeled to the rail. Cool, three fine 30" wide hulls, the Bell and Wenonah ~ 14 feet waterline length, the Swift ~ 15 ft and a little more sluggish in rotation due to length, which also increases carrying capacity.
[For scale, I should mention that WildFire, with a little more bow rocker, much more stern rocker and higher shoulders to lift stems higher spins closer to 270 degrees rotation through my test maneuvers.
Shearwater, at over 16 feet and with less tumblehome cannot lift it's stems to spin, actually skid, hot turns. I've got to really work hard, lots of velocity in, etc, to get past 150 dg rotation. The bow is so far away static draws are less effective. Shearwater cannot lay down through the same roll angle as the others because it has less tumblehome. And, while added length increases eventual speed the extra surface area slows acceleration.
If you keep your boats mostly level, Shearwater will out-turn Merlin in current. But not when laid on its side or better to the rail.