Don’t swim but I do ride my bike for 50 plus kms each ride at a 30km/h plus pace. Once you become accustomed to fasting and your liver is able to convert stored fats into glucose and ketone bodies efficiently you should be able to exercise at about 80-85% (maybe more) of your typical “fully carbed” PB state. Expect this ramping up of your liver performance plus cell metabolism to take several months. It takes time as you are activating a different metabolism path which I can only assume isn’t currently being utilised. Keto adaption to coin a phrase. At a lower intensity it should only take 1 or 2 months.

Since you do laps try going at 80% and see how you go.

Sorry about the jargon. PB = personal best. Since you do laps I assume you know what sort of PB lap times you are able to do? Just go at about 80% of those times and see how you go. My understanding of bio-physiology is limited and my understanding is that accessing stored fats for energy substrate is a slower less efficient process compared to glucose metabolism. Hence you are unlikely to be able to operate at close to your PB state. I ride or go for walks while fasting, and I water fast. As long as the pace is below your ultimate pace then there is no problem. Just give it time and ramp yourself up. Ive been water fasting for 10 months now and it took me about 8 months to feel very comfortable with it. Hopefully it wont take you as long.