A mud duck is a slutty chick that "flys from pond to pond" sleeping with dudes until one or more of them has had a piece and are done with her. Then she flys to the next pond and does the same thing. (The pond being a clique of guys that all hang with each other)
Who's the new chick hangin all over Ryan?
That's Stacy, she's the new mud duck.
Someone not ready for the real world (the big sea -- they're restricted to the mud, not even ready for wading in the shallows). Used by some in the navy as a disparaging term for marines and coast guard. Used also in amateur radio for people whose broadcast is weak (and therefore come through muddy with too much noise and not enough signal).
Wait here, mudduck, and let me show you how it's done.