Sports gels eg High 5 are used by many athletes, not just people with diabetes. Larger than gels specifically aimed at correcting hypoglycaemia, but generally taste better, and electrolyte content of sports gesl might be useful - you'll have to experiment.
I haven't done any long swims, but for a year I lived at Tynemouth and swam / body boarded in North Sea at least an evening or two a week from May to October - with a wetsuit, and gels shoved inside my swimsuit shoulder straps so I could retrieve if needed.
Definitely agree with Pheonix about importance of practicing swimming in open water to get used to how your body reacts to water colder than swimming pool. And Runsweet websire is excellent - both for explanation of underlying principles of physiology as well as practical accounts by real athletes with type 1 diabetes.
Just checked GES website, which says it's freshwater and wetsuits are complusory & hats are supplied (help a bit with warmth and keeping water out of ears) - so make sure you're familiar with swimming in wetsuit and have one that fits perfectly.