This weekend the wife and I did another open water swim at the in-law's house on the lake and I'm happy to report that I did not freak out. How did I go from The Great Freakout to just fine in one week? Mostly through reflection and mental preparation. For example, I realized that most of my irrational freakout came from visual triggers. So, I worked on closing my eyes when my face is down in the water and then open for sighting when out of the water to breathe. I also had all my proper swimming gear this time, so that helped narrow down the things that were unfamiliar. The biggest challenge I still have is with sighting and navigation. I did far better this time, but sighting on something that is in front of me (the cottage on the opposite side of the lake) is proving more difficult than something beside me.
For the purposes of disambiguation, I took some video of the seaweed we had to swim through during a boat ride this weekend.