Waterbody

Dina, by 'marina' if you mean constructed facilities for boat docking, fueling, and associated operations, that would be considered "development" for purposes of SLLp3 Wetland & Water Body Conservation, and SLL Credits 7 through 8. Under SLLp3, Option 2 would apply because of a marina's partial location on a water body, and the only way a new marina could be allowed is by locating its land portion on previously developed land, or by earning a GIBc8 stormwater point and limiting marina impacts to those in Table 1 of the prerequisite. An exception to these requirements is possible if the water body in question is man-made. Assuming either a man-made water body, or previously developed location, or GIBc8 point plus limited buffer impact is possible, the remaining hurdle is the second-to-last sentence of SLLp3 allowing direct impacts to water bodies only through minimal-impact structures like "elevated boardwalks that access water for...recreation purposes." You may want to obtain a CIR confirming that a "boardwalk elevated over water for recreation purposes" includes a marina boat dock.
Eliot

Dina, it's difficult to understand your exact situation without a map, but Option 2 of SLLp3 applies if the distance from the 1st sea water line to the first piece of new construction is 100 ft or less, unless that new construction is on previously developed land, or if the project can qualify for subsection (b) of Option 2.
Eliot