By the way a very good review of the Alchemy is on the wonderful blog you can see here: Woman on water
Lots of boats that are perfectly competent in the sea do not meet the general standard for a sea kayak. The Mystic is too short and the Solstice GT is too wide. There are too many to list.
There are surf ski's that are obviously different than sea kayaks but are they less useful in the sea?
What about the X15 boat the Tsunami use for their touring, camping, and surf shenanigans? It is a 14 feet long and 24 inch wide sit on top. I don't think any ACA instructor I know would call that a Sea Kayak, but it designed purpose is playing and touring in the Pacific Ocean.
I think the biggest reason to group boats into types is to make instruction easier. I was told in no uncertain terms before my instructors course that it was going to be a Coastal Kayaking instructors course and that I must bring a Real Sea Kayak, Sit on tops and Greenland paddles need not apply.
As a very beginner of an instruction I can attest to the problems it can cause when someone shows up with "non-standard" gear. I can show a beginner different stroke with a Euro blade paddle that I rarely use, but I've never been shown any single blade strokes or how bracing is different with a Greenland blade.
The funny thing is that rec boats and sit on tops are outselling sea kayaks and those paddlers are probably not getting instruction because the classes are usually focused on sea kayaks! Maybe the beginning instructor classes could include the basics of all kinds of paddling. In the mean time I'll see if I can get some canoe, Greenland and SUP basics down so I can share the beginning skills with people no matter what gear they show up with.
Rec boat sit on top sailing with a Greenland paddle. I wonder if anyone else is doing this in the salty water?