An extendable and retractable lower 
deck is supported by one or more longitudinally extending hulls. The lower 
deck slides between an extended position and a stowed position on rails that are secured to the underside of an upper 
deck of a houseboat, pontoon boat, tritoon boat, catamaran, floating dock, 
pier-supported dock or other 
watercraft or dock. When fully extended, the floating deck remains attached to the 
watercraft, and additional width may be achieved with laterally expandable panels in the lower deck. Various accessories are connected to the lower deck or to a storage hold in the 
hull beneath the lower deck, such as a 
pool liner insert, a hot tub liner insert, a slide and ladder 
assembly, a seat and mounting bracket 
assembly, a volleyball net and pole 
assembly, an umbrella and pole assembly, a hammock and stand assembly, a seat lift, and an expandable deck insert.