White oak would be fine. I am probably going to be building mine out of the same stuff along the lines of Brettski's floating dock idea, I have a small sawmill and lots of oaks. Red oak has open pores and may not weather as well, water can get into it easier. Oak does lift a lot more splinters than treated pine, so it may not be very good if you like to take your shoes off. Any green wood will shrink so plan for your gaps, they are only going to get bigger.
