I don't know anything about Sea Trekking, but I've done two Swim Trek tours (Croatia and Gozo), in 2007 and 2009, and thoroughly enjoyed both. The guides were wonderful, giving us tips, making us fantastic lunches, and making sure everyone got through the swims safely.
The Tuesday afternoon of both tours was given over to stroke filming in a quiet area, with stroke analysis in the evening. I didn't see much difference between their coaching and that of the T.I. coach that I was learning from at the time. In fact, one of the guides on the first tour started off the analysis by asking us if any of us had ever heard of Total Immersion!
My only advice regarding Swim Trek is to pay the extra for a single room if you aren't going with anyone. European hotel rooms are tiny. Also, add a day or two extra to either the beginning or end of your trip for sight seeing, as the tours are usually in very interesting parts of the world. Oh, and do some hiking as part of your training beforehand. The hikes can be strenuous.