I typically use 12 foot 2x6's. I also use the cheap landscape timbers to provide additional support (2 for each 12 foot span). I will usually nail a piece of scrap pressure treated lumber to the bottom of the landscape timber to add a foot to spread the weight. I also deck the pier with 12 foot 2x6's cut to 4 foot sections. A 4 foot wide pier is plenty unless you have other plans. I also split the two 12 foot joisted with a 2x4 joist to provide center stability. That keeps the deck boards from sagging. I hope this helps. By the way, you can work effectively in water up to your arm pits. Any deeper and it is very difficult.
