I used 2x6 treated lumber. The regular deck boards would probably be ok, I just thought for the price difference the 2x6 would last longer.
My dock uses 8" light I beams for the structure with 2x6 stringers and 2x6 decking. Sets on Osage Orange posts. 80' x 8' main dock with 12x16' floating and 8x8 gang way.
