An expansion deck system for vessels includes a frame assembly for positioning between a stationary floor of a vessel and a plurality of pontoons of the vessel. The frame assembly including a first side with a first elongated opening, and a second side with a second elongated opening. One or more sliding deck members are telescopically connected to the frame assembly by one or more sliding rails and transition between a retracted position and an extended position. In the retracted position the one or more sliding deck members are positioned between the floor and pontoons of the vessel, and in the extended position the one or more sliding deck members are positioned alongside the frame assembly.