Abstract: A combination bed liner and fluid retaining unit adapted for mounting within a truck bed in order to selectively ballast the bed. A top cover exhibits a substantially planar base from which extend a pair of sides and an interconnecting rear. A bottom cover exhibits a substantially planar base and a plurality Of upturned side portions overlapping side locations associated with the top. The top and bottom covers are secured together along opposing perimeter locations and in fluid-tight fashion to permit an open interior defined therebetween to be filled with a fluid ballasting medium.