Most of my kayak experience has been in the surf, but I did do some canoeing decades ago in conditions like you describe and found it very pleasant. My take is that enclosed cockpits are beneficial in very rough water like surf or rapids where you you want a low center of gravity for stability and manueverability, you do not want the boat to take on water like a canoe would, and where you may get flipped and need to be able to eskimo roll back upright. For the calm conditions you describe, I do not see an enclosed kayak as being of much benefit, as well as being a pain to get in and out of. Also being able to do a competent eskimo roll is pretty much a prerequisite to spending any time in an enclosed cockpit. As far as canoe's versus sit-on-top kayaks, my feeling is that a double bladed kayak paddle is more than twice as efficient as a canoe paddle, so if the pace you paddle at is at all important, my experience is that canoes are the slowest. Kayaks come in quite a range from big heavy plastic beasts that weigh a ton and are not very hydrodynamic, to lightweight fiberglass kayaks that are sleek and very efficient through the water, but usually quite a bit more fragile. I think that the latter would be the best if part of your goal is to get exercise and to get a good workout. Once your hip is up for it, the other option to consider would be a stand up paddle board. The huge advantage of this is the balance and leg activation which makes for a much better whole body workout than sitting down. I do a lot of stand up paddling in rough ocean conditions, and am really looking forward to getting back into that as part of rehabbing my new hip after a number of months.