I would like to build a small and very basic pier to allow easier access to a small lake. It only needs to go out about ten feet at most. I’ve built decks before, so I’m comfortable with most of the techniques, but I’ve never tried to sink posts into standing water. Most of the sites on the Internet make it look pretty complicated.
The water level where the posts would go is about 6 - 8 feet deep and the soil is muddy.
Any advice?
Use a floating dock, there are a lot of plans and kits available.
You’ll probably need to add a bit more information. Is this a permanent structure? Will it be removed during winter? Is ice going to be a consideration? Will a boat, or boats, be docked there? Is high water (flooding or tides) a consideration?
Are you considering a mostly wooden pier, floating plastic logs, a metal frame with wood planks?
Large concrete or rock structures can be positioned during ice over. They will drop into place during the spring thaw.
also check your local regulations and permits needed. there could be state, county and town/city/village regulations
It would be permanent. The location is Texas so ice is not a problem. It’s a small lake on private property. Flooding and flowing water is not an issue. Just something very simple to keep from wading through mud and yuck to get to the water.
You could start dumping the largest rock you can find at the end point of proposed structure.
Cheaper and (possibly) quicker than driving pilings or emptying the lake and removing mud until you find a solid layer of something.
Or you can build a quick and cheap floater and know that it will wear out in 50 years.
I have never sunk a post into mud for a pier but I love to guess at these things.
I would use a hose as a water jet. Strap the hose to a sharpened 4x4 with something breakable so you can retrieve he hose.
I’ve seen a newer pile driver that vibrated the piling instead of hitting it. If you have a heavy pneumatic tool you could strap to a sharpened 4x4 and the combination of weight and vibration may set it.
I’m guessing that there is water where you suggest driving a wooden stick (the sharpened point will negate any pressure treating it may have). How does one hold a 4x4 upright in water, and how do you reach the top of it to drive it?
It will need to be as long as:
Alternately, you could start with a stack of 4x4x8 with coupling sleeves, then stand on the pier you’re building to start sinking them until one ends up above the water…
Care to guess again? 
My first thought was the same as TriPolar’s…get a floating one. I’d imagine you can just buy them online or at a marine store (which you probably have nearby if you live on a lake and people have piers).
If you need to set piles, I’d probably leave that up to the pros, I’d hate to do it myself and have them shift and dump someone in the water and leave you responsible. I assume permits would be required for a job like that and I wouldn’t be surprised if the DNR had regulations for how to set them.
OTOH, you can get a couple of these for 100 bucks or so and build a deck right over the top of them like this. It should be a lot easier then trying to set piles into wet mud underwater.
(Googling and searching Amazon for ‘dock float’ brings up all kinds of results)
Wood is pretty resistant to rot when sunk deep into mud. Heck, it seems like every few month they discover a 5000 year fish trap made out sticks driven into mud.
Neither of my options required that you reach the top of the 4x4 to drive it. I specifically proposed using a water jet or vibration for that reason.
As for actually holding the 4x4s I would tie two john boats together side by side. I’d block out a 6x6 gap between the boats and run some lines to shore. 
Um, both shores, no? Fore and aft, to keep the lines taut? Now, about drifting sideways…
Now, to keep them perfectly level, so the end of your 4x4’s don’t wobble around…
A pontoon boat with large anchors in at least all 4 primary compass directions and come-alongs to tension them might get a floating platform.
And this is a pier - lots of weight 7/24/365, and maybe an unspecified weather event once in a while. Either your 4x4 is resting on something a wee bit more solid than mud or that pier will self-destruct long before a floater would.
Back when the pontoons for a floating dock had to be steel, you had to figure on rust out. Modern plastics can replace both the pontoons and the deck. Splurge and use stainless for and small hardware you must use, and you have a permanent structure.
I quick google on “underwater concrete” will give you an idea why we are saying “floater”.
Where else does one build a pier? 
I just built a new one at my house a few years ago, Mud/sand soil down several feet. The pilings went in super easy, actually. The method I used was to get a gasoline trash pump, 3 inch inlet, 2 inch outlet, and then reduce the outlet down to a 3/4" PVC pipe. Frame the piling a bit if you can, set it down on the bottom as near vertical as possible, start the pump, and blast the dirt at the bottom out.
The piling should just plain sink in at that point. Rocks and roots can be a problem, but with a little jiggling, you can usually get around them. All 20 pilings for my dock were placed this way, and even though there were some roots, I had no problems getting each one set to at least 4 feet, and some went to 6. Total time to flush one in was about 5 minutes.
I bought an inexpensive pump from Amazon, for around $300, but I’m sure you could rent one for a weekend for much less.
Send me a PM with your email if you like, I’ll try to see if I can find any of the videos I took of the process.
— ETA:
I used 6x6 treated for all pilings, 2x8 treated for stringers, 2x6 treated for joists (all joists hung with hangers), and topped it with Trex decking board. It is more solid than my house.
Yeah, I know. I strive for meaningful titles.
Khendrask, why don’t you post your videos to YouTube? I think a lot of people would find that useful.
All things considered it looks like a floating dock is the best way to go. I’m a little surprised that piers are so problematic. They’ve been building piers since there have been boats, so I wonder how the old timers did it.
In your situation I’d buy an old pontoon boat, strip any remaining seating, and use it as a floating pier. Alternatively, a floating dock using barrels like in this instructable.
You seem to know a lot about this. Perhaps you could enlighten me as to how Khendrask wasn’t able to use a water jet to set pilings for a pier?
