It depends how you react to exercise. Some people find it makes their BS high, but I find the opposite, so I start my swim with a good BS, topping up with a few glucose tablets if necessary.
My advice is to tell the lifeguard you have Type 1, put your meter and glucose by the pool, and stop to test at frequent intervals. I find swimming burns off a lot of glucose personally, but you'll need to find how it affects you.