A strap apparatus for holding a foldable boat seat including a seat back pivotally connected to a seat base in a folded down position is provided. The strap apparatus comprises an elongated strap body that is made of a material having a high stretch resistance, a first fastener for attaching the strap body to the seat base, and a second fastener for removably attaching the strap body to the seat back such that when the strap body is attached to the seat back, at least a portion of the strap body extends between the first fastener and the second fastener. Strap adjustment apparatus are associated with the strap body and function to adjust the length of the portion of the strap body that extends between the first fastener and the second fastener when the strap body is attached to the seat back and thereby allow the seat back to be pulled down tight against the seat base. A foldable boat seat and a method of installing a replacement foldable boat seat strap on a foldable boat seat are also provided. The adjustable strap apparatus is also useful in general marine and other applications.