Turtle Mommy,
True driftwood, according to my own definition, is a naturally fallen tree that has gone through several phases of evolution, or rather, processing to meet my criteria of good quality driftwood. I'll expand on that "criteria" later on.
1] The tree/branch blows down in a wind or ice storm, a beaver cuts it down or a flood washes it out of the river bank.
2] It is carried downstream by the current and smashes into rocks, ice or other obstructions and all the bark is removed.
3] The water soaks the wood and leaches out most of the sap.
4] Minerals from the water begin to migrate into the cellulose structure of the now dead cells.
5] The wood comes to rest out of water on a dry sandbar for a summer period where the sun bakes and dries it.
6] It continues downstream again, going through another year of being smashed and banged and washed and dried.
7] It turns out looking gnarly, smooth textured, lightweight and hard, not water-logged and not soft (spongey) or rotting.
Best places to find excellently conditioned driftwood such as this are either on an ocean beach or in a slow flowing, sand or gravel bottomed river. You will know it is a good piece if it looks huge and should be heavy, but it is amazingly lightweight.
The reason I have this criteria for driftwood is because when I was a teenager, my sister-in-law was into arts and crafts. One of her many projects was to take a piece of driftwood and stain, shellac or paint it and mount miniature figurines of small animals and plants or flowers upon it or drill holes in it and make a candle holder out of it and sell them or give them as gifts. The driftwood had to have a specific shape or design and be "cured" by nature's elements so that it would not be just a rotting piece of timber. Since I was practically living on the river all the time, she asked me to collect such specimens for her artwork. So I developed a keen sense of what specimens were the best.
If you find specimens such as these, they will last a very, very long time out of water (decades). If you put them in your pond water for the turtles to roost upon, they will eventually begin to rot, but should last much longer than just a normal piece of tree limb that has not undergone this processing or evolution. They actually become cross between water resistant cork (lightweight and bouyant) and being petrified or preserved, but not heavy like a stone.
Gordy