Not that this is a big effect in the ocean, but still, it’s a nice demo.
A body submerges into the water until it displaces an amount of water that is equal to its own weight. So if you have a ship with an anchor onboard, and then you drop the anchor into the water. Will that affect the water level? And if so, how?
I like this demonstration because it is so easy and also because so few people get it right when you ask them about it (which is actually a bit shocking).