When an object (say a boat) moves through water, there certainly
is a type of friction. The layer of water near the boat at least partly
moves along with the boat, and rubs against other parts of the water.
The water exerts a net drag force on the boat, and tends to make the
boat move along with the average water flow, or stay put if the water
isn't going anywhere. The water will heat up slightly as energy from
the big motion gets lost in energy of the jiggling water molecules.
This friction in liquids is different from friction between solids
in an important way. Between solids, friction can be big when they
aren't sliding and drops some when they start to slide. In a liquid,
the faster something moves through the liquid the more friction there
If the boat isn't streamlined, it also has to push the water out
of its way, increasing the drag force. The shape of the boat is really
important here. The more streamlined the boat is, the less will need to
be pushed aside. When the boat starts going fast, the force needed to
push the water aside can become much larger than the
force needed to drag the water along the sides of the boat.
The deeper you get underwater, the denser the water gets. That is,
further down, the water is more packed together - the molecules are
more closely crowded. I guess a little increase in density should
affect the water's viscosity- how hard it is to drag
things through it. If you go really deep, that packing increase is
enough even to noticeably increase the water's density itself, so
there's more to push aside. So the deeper down you get, the more force
there is resisting the boat's motion. To find out more about the math
involved here, you can look at the answer to Calculating Underwater Pressure.
(republished on 07/31/06)