I swam 1 km tonight.
Accidentally did a full bolus for dinner, even though I'd intended to reduce it. So before swimming I ate 20g of carbohydrates with no bolus. I didn't reduce my basal at all.
BG was 8.8 (158) before starting, and 4.7 (85) after finishing; it took me 35 minutes to swim the 1 km (mix of freestyle and backstroke).
I messed up with the bus schedule so had to wait an hour for the bus. Luckily I had a book. So an hour and a half after exercise I'd risen to 9.4 (169) without eating.
Next time I think I'll try bolusing a unit after I finish. At least this time I didn't spike nearly as high as I have before, but probably also because I ended lower.