Hey deerman, I used 2X6 pressure treated for my runners and 2X8 pressure treated for my decking. For sinking 4x4 into a pond that is full you can do one of 2 things. ether you can cut the corners off the end you want to sink making it like a spear and drive it, or you can use a water or in some cases a high pressure water hose next to it and it will blow a hole for the post to go down into. Here is a link to my dock build I just did a couple weeks ago.
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=448338&#Post448338


[Linked Image from i.imgur.com]