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As to your other question, the number of posts depends on the location of the deck. Assuming the one edge rests on the shore, then you should only need 2 posts to support the ****hest edge out oveer the water. If your deck is entirely over the water with some other form of access to it, then you'll need 4 posts. One at each corner, with knee bracing back to the deck frame in either case.
And yes, you can make you posts concrete from top to bottom. The key will be to install the tubes with sufficient bearing and bury at the bottom so that the weight of the concrete doesn't make them sink at all. They must bear on good comapcted gravel. The tubes must be the right length. And you'll need to build a framework to support them in place until the concrete sets up. The framework will need to keep the tubes plumb and level. Once they're filled with wet concrete, and before the concrete sets up, if they begin to settle or shift or twist or fall out of plumb, there's very little you can do but remove them and do it all over again.