DS2 has been sick the last two occasions that we've been swimming (several months apart). Not due to illness, just swallowing too much pool water. DS1 used to swallow his share of pool water, but I don't think he was ever sick.
I'm not really sure how to stop him swallowing water - he wears a float vest and splashes around fairly independently, with his face generally out of the water. I discourage DS1 from towing DS2 around and other activities that could make him swallow water. But it seems that he swallowed loads of water today - when he threw up, it was gushing water, with the odd bit of partially digested food (sorry for TMI).
Apart from making sure that we leave it as long as possible after eating (it was probably about an hour after lunch today, I'm thinking we might need to leave it a couple of hours), are there any other tips to stop this happening?