So when your rowboat is floating, it can displace a certain volume of water and if it displaces more than that volume the water spills over the sides and it sinks. We talk about how many tons of water it’s displacing because that tells us what the total weight of the boat, you, cooler, beers, tackle and oars can be before the boat sinks.

You already knew that though. What might not be clear is what that weight measurement actually is.

Weight is the acceleration due to gravity that an object experiences. So if your rowboat is able to displace a volume of water that experiences more acceleration due to gravity than it and all it’s contents do, it will stay on top of the water in a state we call floating even though it and some of the contents may be more dense than water!

Now your rowboat is different than the balls in that floating glass ball thermometer your aunt bought out of sharper image in one very unique way: it can’t function when submerged! Those little suckers will go up and down all day, but once water starts coming in over your gunwales you gotta get rid of it or the boat sinks and won’t come up.

So there’s a point of no return where your boat can’t stay afloat any more.

When it displaces a volume of water that experiences less acceleration due to gravity than it and all its contents do, it and all its contents are pulled under the surface of the water. At that point, density determines what happens to the boat and it’s cargo. The boat itself may be denser than an equivalent volume of water and sink, but the beers and cooler are less dense than water and they float. You may be more dense than water, but instead of sinking you tread water and push your head up above the surface.

When the swamped boat sinks, it experiences more acceleration due to gravity than the water around it and pushes that water aside on its way to the bottom of the lake. The beers experience less acceleration due to gravity than the water around them so the water is pulled underneath them and they float. The air pocket inside each can also lends some displacement to the cause.

So the volume of fluid displaced isn’t “slight”. It’s exactly what gravity itself requires for objects to sink or float!

Gravity likes water even more than it likes boats.

It's actually interesting because when you consider the four fundamental forces of nature (gravity, electromagnetism, strong nuclear, weak nuclear) gravity is by far the weakest one. It's not intuitive because gravity is the one we interact with most day to day and it has connotations of large objects like planets and stars. But it's only a significant force when you have such large objects. Two magnets technically are gravitationally attracted to each other (like all things that have mass) but it pales in comparison to the magnetic forces.

Gravity is pretty weak, all things considered. A tiny magnet can easily overcome the entire Earth's gravity on a small metal object.

Your comment has made me realise I don't understand how floating works.

The cool thing is that they're floating because of gravity. Specially, the thing they're floating in is heavier than they are, so the float medium gets pulled underneath the object.