Just as a side note - most swim schools wont take kids until they are 6m old. This could be for 2 reasons - they have had most of their jabs by then, and the breastfeeding poo is being supplemented by solids in most cases, so wont be so runny.
I have only ever seen one baby do a poo in water, and that was a 1w old baby held by Laurie Lawrence on a Huggies tour for water safety. Considering the nappies were sponsered by huggies, and it nowhere near fitted on the newborn, there wasn't a hope in hell of holding in the poo. It just mixed with the water and it all went brown.
By the time they are 6-9m old, I think the distraction of the water and what you are doing holds off pooing for some reason.