Salt is a mordant (fixer) for certain organic dyes. It is ***not*** an effective mordant for the newer, chemically-based dyes. Quilters today are using fabrics that are processed using chemical dyes, so rinsing in salt water is more a bow to old ways than an effective technique.
The one thing that is effective about rinsing in the ocean is that you are *certain* to have sufficient water to disperse any dye bleeds!!! Some fabrics are not rinsed sufficiently by the manufacturer and thus have excess dye in them (more dye than the fibers can absorb). This excess dye would be effectively dispersed by rinsing in the ocean. However, if you have a fabric that is a real bleeder (the dye was not set properly by the manufacturer) it will still bleed the next time it is washed.