How long does it take to waterlog a log?

Looking at a natural swimming pool that would fit in an urban yard, I'm thinking you'd probably need walls for the swimming area that are nearly vertical. These walls are going to need some sort of stabilization. So far I have seen tires and I have seen cement. I'd like to stay away from both of those options. Talking to Paul about it, he came up with the idea of burying logs vertically with their tops below the water line to act as a stabilizer for the walls.

One problem is that wood likes to float. So in order for this to work the logs would need to first be waterlogged. I can't say that I have intentionally tried to sink big logs. Does anyone have any estimates as to how long it takes to waterlog a log?

While I can't answer your question, I do have an idea:
What if the log would initially extend above the water level, so the above-water part balances the below-water part? It can then later be cut off. (temporarily lowering the water table?)
If a fairly dense wood is used, I would assume that the friction of the soil around it should keep it under water.

There are a number of trees whose wood is so dense that it sinks in water.  Many of these trees have a common name of "ironwood".  In the southeast USA, Carpinus caroliniana (Carolina ironwood) holds this honor.  Any wood that weighs more than 62 lbs per square foot will sink in water.

Here in Canada, Shagbark Hickory is a definite sinker. At one time I had a beachcombing boat for pulling logs off the West Coast where I live now. Red cedar takes just about forever to sink, while hemlock can sink in saltwater within a few months. Speed of decay is reduced in relatively clean oxygen poor water. Probably the best choice would be black locust because of it's resistance to decay. It's also a dense wood that will not have buoyancy.