Swim nappies advice!

We're starting swimming lessons in a few weeks and I am ... nervous about the nappy situation. Wee guy is 4mo and doesn't poop often (once every 4/5 days) but when he does it is An Event. Usually requires full clothes change, immediate bath, the lot. His poop is still mostly liquid, and I am concerned that if we're swimming, a swim nappy will not hold in it and I'll be watching in horror as the water around us turns yellow. Anyone got any advice on navigating this??