What were they made of? Only Wood boats actually Float, Fiberglass, Metal, Cement Boats can't(they remain on the water by Displacing an Amount of Water equal to their Weight), and are required by Law, to have adequate Buoyancy added to keep a swamped boat from sinking. Some boats use Sealed Chambers for Air, others use Foam.
You might not just have seen any Floatation, but that doesn't mean the Boat does not have it
As for you thinking that the kind Water the Boat is used in... It makes no difference on whether it floats or sinks.