We are installing a platform/dock on a newly built freshwater pond. The property has a great amount of red cedar available for our use. How long could we expect red cedar posts to last if used as pilings? How much longer would black locust last?
I can't tell you exactly, but they don't last as long as you'd want them to. We use eastern read cedar as horse fence posts through the woods, occasionally. I've had them go down in as little as 8 years. Others last longer.
Having recently had to repair a dock, it's worth it, due the large amount of physical effort it will require to install the posts, to use quality posts. Not to mention the hassle, if they begin to rot, of removing them. You'll be much better off using a treated post. If you can find them, creosote telephone poles would probably be ideal in price vs quality.
There are only two issues with them. One is that I believe they've outlawed creosote wood. I don't know if that's just in my state or nationwide, but I know you can't find it around here any more because they just don't do it. I don't know if it was toxic or what, but even with railroad ties, I believe they're going to compressed recycled plastic ties.
The other issue is that the telephone company might not allow you to buy them, even if they have them. They used to let you buy old telephone posts as they replaced them and my grandfather built many fences, barns, and even the posts for our pier out of them. Some of them have been in the water for 50+ years and are still solid. However, they decided to quite selling them to the public in the last few years around here, because they decided that people were using them to build structures and they didn't want the liability of someone building say, a barn, with their posts when they couldn't guarantee there structural soundness, and have the barn fall in. So they just stopped selling them to the public.
If you can't find them, I would go with a treated post of some sort. If it's a small dock on a pond, you might get away with treated 6"x6" posts.
I would also suggest you look on youtube.com for one of several videos where people make homemade pile drivers. I saw one guy from Australia who imbedded a chain in a 5 gal. bucket he filled pure Portland cememnt. He imbedded a piece of .25" metal on the hammer side for some extra support. He then made a block and tackle rig with 2"x4" that he would attach to the post to be driven. Helpers in the water would use guy lines to keep it straight and others would lift and drop the "hammer". He drove a large post with considerably less effort than I was able to drive my much smaller posts in. I think it would be worth it in the future for me to make one of these. Good luck.