Water resistance is another word for friction. When you move through water, you have to push water out of the way. As the water passes across your skin. it pulls against it, tending to make you stop.

Because water is quite a dense substance, water resistance is high. This is why ships and other things in the water have a blade or cigar shape. Most animals in the water look like this. Most of the things we design to travel through water are a similar shape. Ships have a v-shaped bow to 'cut' through (part) the water with as little drag as possible.

The drag can be increased greatly if the surface is rough, so most ships are serviced regularly to clean their hulls of barnacles and other things that grow on them. Unless this is done, the fuel used getting through the water increases greatly, and the ship's speed is lower, too.