I swam in my estuary till well into Autumn. The water temp dropped from a comfortable 18C to 15C. I love the feel of the cooler water but I have to limit my time to no more than 30 mins or I get cold to the core and take forever to warm up. Could be something to do with lack of insulation and age. In the water if I overdo it I start to lose coordination, a bit like when you realize you are biting your snorkel mouth piece and know it's time to get out. What really finished my season was the half day it took to warm up, even after hot showers. That and Merle worrying about her mad husband. Curiously, I joined the local mid winter swim at the beach on a truly horrible day. The water was about 11C with a nasty shore break. I swam out past it and was revelling in it then looked up and realized that everyone else was headed for the pavillion. That was about 5 minutes immersion and no ill effects at all. I am not going to try Naj's + 10% trick but now the days are lengthening I may don my 2mm sailing wetsuit and see how it is..
All the best