I just checked out Tarrafal swim. It looks fabulous. (I am fluent in Spanish so reading the Portuguese is no problem). Swimming is great because you just don't need much. Though I have weak eyes and so need goggles to protect my eyes in open water. I love you approach and great that you can have a boat around to support people. It reduces the risk and also give new people security to try stuff they might not otherwise.
My experience is that any swim drill that works in a pool can be done in open water, as long as you aren't pushing yourself to the point of exhaustion. (which I don't think is particularly productive anyhow)
I am pretty new to open water (only a couple of months) and I find the head-game of it is really important. You can't breath effectively when you are freaking out and if you can't breath you can't swim. As you get more comfortable you can start doing more 'drill' (Such as increasing the pace for set distances, fist drills, one-arm drills etc) but nothing is ever as satisfying as just getting in the water and going.