Water isn't completely incompressible but to compress it you need so much pressure that often we approximate it to be incompressible.
Having said that there are still a few differences. The incompressibility of water makes it resistant to changes in volume but it can still change in shape. If you fill a plastic water bottle completely with water you can't really change it's volume. I just tried it and the water bottle could easily hold my weight. It flattened a little because the easily deformable bottle could become flatter without changing its volume (although there is no easy way to test if the volume really stayed the same). You could counteract this by increasing the pressure of the water but this is cheating because a high pressure gas also acts more like a solid. A soccer ball that's under high pressure also almost acts like a solid.
Another difference is that water can move freely. A fully filled water bottle can spin more easily around its axis than a bottle filled with a solid of the same density. This is because initially the walls of the bottle can spin without the water spinning along with it. Generally the water can have all sorts of crazy currents inside which can affect the rotation of the bottle. These currents don't influence the center of mass though because the density of the fluid is constant. When there is air inside sloshing can happen and this forms a big problem for trucks carrying liquid or for spacecraft. The sloshing makes braking harder and also makes it harder to maneuver and that's why there is a lot of research in slosh dynamics.
So in conclusion water in a closed container behaves a little like a solid but it is still fundamentally different. It is resistant to changes in volume but can still change shape (think of water balloons). Because it is completely filled it shows no sloshing but it still behaves differently because the fluid is able to move freely inside.
Oops I just noticed this post is over 3 years old.