I went swimming both last night and the night before.
The night before last my brother decided to join me, since I was visiting my parents. That was really nice, and I can't imagine would have happened 10 years ago when we were basically both teenagers (well, I would have been 20). My blood sugar was 12.5 (225) before starting, and I did a 1/2 correction and then was 5.3 (95) when finishing (swam 1 km), but was up to 7.4 (133) within the next 30 minutes, which I corrected for. The rest of the night was okay, and I woke up at 3.1 (56) the next morning.
Last night I started out high again at 13.3 (239) which I did not correct for at all. I only dropped to 11.8 (212) after swimming 1.5 km, which I corrected. An hour and a half after swimming I'd spiked to 17.2 (310) and blood ketones had gone up to 1.8, which is getting pretty high. Anyway, I corrected that and went to bed when my blood sugar was down to 11.8 (212) and ketones were 0.9. I woke up at 3:00 AM at 3.6 (65) and ate four glucose tablets, and woke up this morning at 6:00 at 4.3 (77).
I talked to a friend who is a swimmer with Type 1 and said this same thing with spiking BG and ketones happens to her, and gave me some tips fro dealing with it. So if I swim tonight (probably will) I'll try some of those out, but I also welcome any ideas people here have!