when i was youngER (still young.. just a teen). i heard stories that they could swim.. This was before anyone ever told me "They'll sink". so i put a hen i had in the pool with me.. Just in case she couldn't i stood next to her closely.. And she swam for about 20 seconds and slowly started to sink, after that i grabbed her and put her out of the pool.. So pretty much until the oil washes off their feathers they can swim.. But after that, They're goners.
Our neighbor's have those huge Maliays and Aseels and I scared a rooster once and he swam all the way across their lagoon easily and after he just shook his feathers off and walked off.