If you do not brace your posts with diagonal cross bracing, the deck with wobble on you. You will need to make an X from post to post at the end and on both sides at the very least. Maybe more depending on what you use for posts and the size of the deck.
Concrete isn't going to remain solid inside a pipe if it's moving around. My guess is that it will crack inside the pipe almost immediately and just become filler without any structural significance.
If you were dead set on using pipe, you might consider adding T's to the posts. I don't know if this will work, but think it might if you connect all the posts together about mid height.
With good bracing, it really doesn't matter how deep into the ground your posts are. Depth for posts is dependent on what you are building. For fences, the concern is with heave and wind load. Usually 2 1/2 feet is plenty for a 6 foot fence, but in some soils, you need 3 to 4 feet. For a pole building around here, 3 feet is good, but 4 feet is even better. For a deck with good bracing, I go down two feet, or until I hit rock. In soft sand, I add concrete to my posts, but in clay, I use the clay to back fill.
For the pond, all you need to do is get down to a solid base. A concrete cookie at the bottom of the hole would be a nice bonus, but you're goal isn't wind load or heave resistance. It's to create a solid base to support the load of the deck.
Or you could just go up in size until you get to a post thats thick enough to withstand any lateral movement. What that is in PVC, I have no idea? LOL
If you go with wood, be sure it's rated for water contact. Most pressure treated wood that you buy at the stores will be rated for contact with the ground. Post for piers have a higher level on chemicals in them for constant water contact. I know allot of guys use the posts they buy from Home Depot or Lowes without any problems, so it might just be one of those things that you have to make the call on.
Good luck,
Eddie
