@Daisy_Mae, my T1D son swims a lot every week in swim team. Normally we try to have little IOB when he swims, so he tries to have a meal 3.5 hours before, but it is rarely possible. When he has a meal up to 1.5 hour prior to swimming, we make it a small meal so that there is not too much IOB, but we use normal bolus dosing (not low dosing). We found out he runs low a little faster with IOB. He goes through pretty much exactly 15g sugar every 20 minutes when swimming. He goes out of the pool every 20 minutes to test (or to get a beep from his dexcom), then eats 15 grams and goes back in, for 2 hours on a row.
Before every practice, one half hour before we start bringing his sugars up to about 150 (180 he is has IOB). Then we replenish all the way through. At the end of the practice, we taper down to roughly 100 if we are able to. Then, right after practice, we replenish with 12 oz whole milk, and sometimes more (and we dose). He is on MDI - no pump.