Did it look something like this?
Hee-hee, happens all the time with tankers - that's actually realistic, they're called tankers because they have lots of tanks. If it doesn't blow up or catch fire, empty/partially filled tanks will keep it afloat, if it's in ballast (empty tanks) you have to let the air out of a lot of tanks before it will actually sink. Next time it happens check if the superfloaters are tankers, good bet they are.