See summary. Illager beasts are currently portrayed as bulky, heavy monsters, yet when in water, it "swims" like several clearly far lighter mobs, such as pigs. Its equivalent, the Iron Golem, on the other hand, sinks in water. As both seem to be portrayed as powerful, heavy mobs, it makes no sense for the iron golem to float, but not the equally-sized beasts.
(If I get yelled at for "posting a suggestion," I just want to point out (again) that this is just something that doesn't make sense at all.)