You are really considering boats that are at such extreme ends of the spectrum that I have to wonder how well you are defining your goals for these boats. I also have to question your preference for an aft cabin in a boat this size. Having designed quite a few boats of this size, I strongly believe that center cockpit layouts really so not make any sense in boats under 40 or so feet and that they really don''t come into their own until a boat is roughly 42 feet or so. To explain, when you talk about a 36 footer intended to go distance cruising, every bit of space becomes critical for storage and living space. On a any boat, but especially a boat under 40 or so feet, the area near the roll and pitch centers of the boat become especially critical living space because of their decreased motion. This is also generally the widest part of the boat; the area where it is easiest to layout a workable salon and galley.
When you place the cockpit in this area of the boat a number of detrimental things are forced to occur. It forces the primary living areas towards the ends of the boat and it forces the cockpit to be raised making offsetting any motion comfort that might otherwise occur by placing the cockpit in this prime real estate. Pushing the main living areas to the ends of the boat increases the amount of circulation area (walking area) and circulation is not useful for anything else such as tankage, storage, or work surfaces. To get around this either the hull form needs to be altered in order to provide adequate space for the accomodations or else the accommodations and storage get miniaturized to make room for all else that is necessary.
Getting to the specifics of the boats in question:
CS 36: The CS 36 is a very nice boat. They are well constructed and offer excellent performance. There are actually two very distinctly different CS 36''s. Of the two the Merlin is probably a better choice. If I have a gripe with the earlier boats, it is with their IOR era hull forms and rig proportions. Still these are very nice boats. On the other hand, I am not sure that they are really very good liveaboard/ offshore cruisers, seemingly more optimized as performance coastal cruisers.
Contest 36S: To some extent, the Contest 36S is the exception to my center cockpit in boats under 40 foot rule. The layout on the Contest seems to work reasonably well. My family owned an early Contest as their forst boat and so I have had a strong interest in Contests ever since. Contests tend to be very well finished and offer nice performance in a breeze, but have never struck me as being terribly robustly constructed. (Ours certainly wasn''t requiring a lot of structural repairs for a comparatively new boat.)One example of that is Contests use of mid-topside hull to deck joints. Although an inexpensive way to build a boat, a mid-topside hull todeck joint is very vulnerable and it is very difficult to get sufficient flange area and stiffness at this critical juncture to prevent damage or premature failure of the joint.
S2-11: While S2 has a strong following, I have never been a fan of these boats. My exposure to these boats has left me with serious questions about the build quality, and in the case of the 11.0, I have not been impressed with the sailing ability as well. By and large the S2 11.0 is a good example of what I don''t like about small center cockpit boats.
There are two very distinctly different boats called a Mariner 36. The one was an Oriental built, alledgely Alden design that I would run don''t walk the opposite direction from on almost all counts. The other is a New England built boat that reportedly is a pretty nice boat.
Watkins 36: I have nothing even slightly good to say about these boats and so will just plain keep my mouth shut.