Abstract: A decking system for a boat includes a deck substructure composed of aluminum cross members attachable on pontoons of the boat, and elongated aluminum deck planks being attachable to the cross members of the deck substructure in side-by-side relation to one another and thereabove to provide a deck platform upon the deck substructure. The planks include a pair of planks at outer sides of the deck platform and a plurality of middle planks therebetween. Each outer side and middle plank has a main platform-forming portion with a pair of opposite longitudinal edges thereon and plank-supporting rail and runner portions depending adjacent the edges of the platform-forming portion. Tongue and chennel portions are defined along the respective edges to provide mating between adjacent ones of the planks.