In any rough chop and high winds expect to get wet unless you go it slower. A good spray skirt with an implosion bar will keep you dry. Dress accordingly of course. I took mine out recently in high 20-35 knot winds (Small Boat Advisory) on the Indian River here in Florida. The chop was wind driven and 2 foot at the mouth of the turkey creek with a rough 2-3' fetch in the middle and smooth waved swells on the edges of 2+ feet at the mouth where it meets the Indian River. I stayed dry if I was smart about my course and tacked a bit and kept my speed down rather then plowing through. My Merganser has no rudder or skeg and I had trouble staying on course with quartering tail winds in these higher winds speeds with the weather-cocking toward the wind stopping once I was 90 degrees to the wind. Going 90 deg to wind was an up and down experience in the 2 foot swells and chop but I stayed dry and I felt fully in control. Into the wind there were no problems from 90 deg to zero. If you expect allot of rough marginal paddling I'd install a retractable skeg.
In moderate 10-15 knots or less and/or smooth conditions its a smooth dry fast ride and carved turns on the secondary are crisp. I am easily able to maintain 4-5 knots and can hold any course with well practiced corrective sweep every so often or other strokes used to advantage. The Merganser always answered. If your use to rudder turns carving a turn takes more radius especially at speed but is less drag and takes no extra effort to maintain momentum relative to rudder turns.
On winding streams and rivers the Merganser performs very well for me as long as I hold the speed in check relative to the conditions. It often wants to go faster then it should for the tight winding streams but responds well in up to a 4 knot current and I can sit all day facing up or down down stream holding position with very little effort doing so. The sharp exit at the stern allows this, plus allows me to paddle backward into the current effortlessly as long as I'm careful about my alignment to the current. Its fun leisurely paddling backward in open streams or waters when on a group trip all the while talking with my friends. Try that with a ruddered boat! Again ones experience sets the limit. Of course in very tight conditions a 12' or less yak will excel over the Merganser.
I don't have enough experience to evaluate my merganser fully in Surf conditions yet. I've Surfed the 2' wind driven waves in the river successfully with the Merganser accelerating well and I was able to use corrective rudder strokes etc to ride along at up to 7+ MPH according to my GPS but my one time ocean experience was not a good evaluation due to me not having a spray skirt at the time and the 4' waves were far bigger than I'd experienced previously and at that time I had no surfing experience. It felt tippier in the ocean and rode higher in the water perhaps due to the salt water buoyancy effect. Now that I've got a good skirt I'll be putting my helmet on and trying moderate ocean surf.
Lastly I've slowly been improving the fit of my merganser to my needs. I still need knee braces and have quickly discovered ever bit of fit and snugness added dramatically improves my ability to get the most from my Merganser. It keeps getting better and better as my abilities improve.