Sorry so late to the thread, but considering the weight you are planning to put on them, the pvc pipe idea is not good idea. They would have to be sunk to a level for end bearing and you would have a tough time getting them there. Treated wood piles would last a long time. But if he wants the best, some 10" H piles would be your bet, coated with an epoxy before driving, you can buy them that way. Also, the guy that could drive the wooden piles should also be able to do these with a new driving cap that you may have to buy. Concrete piles or any larger pile would in no way be cost efective. The weight of even a small concrete pile and the equipment to drive im sure he does not even want to pay for. Keep in mind, its really only at the water survice and above that a steel pile needs protected. The amount of piles is based totally on the weight of what you are wanting to put on top of them. For most part, piles are thought of as end bearing, as in tons or kips. In general, find out how much the building or boat is going to weigh, know to what bearing you are going to drive the pile to, your pile driver should be able to tell you what he thinks he should get, divide it, and it will tell you how many you need, figure on a 2 to 1 safety factor. Keep in mind, depending on how far they extend out of the water, lateral forces will come into play. This is a whole nother thing, believe me the wind load on the side of the building generates large amounts of forces. Depending on the amount out of the water, even the way the pile are turned comes into play for lateral loads.
