Wel, there are still 3 Morocco ports and Ile de Goree in Senegal (as present). And there is more to do in Tenerife than the beach, at least a chance to walk around in between sea days. I was on three of the islands in December, very different, Lanzarote very dry (closest to the Sahara), and more lush as you go West. Port congestion? Must be commercial traffic, not many cruise ships go there.
Why are the expedition ships "not the best for sea days"? Not enough to do, too unstable? It's my first time on Seabourn's expedition ships. Also have another booked for the fall, so interested why you mention this.