If you search the forum threads by using the search function (the magnifying glass at the upper right of the Mavic Forum page) you may find info on this.
I can tell you I am not aware of anything that is commercially available out there in the line of water landing gear for the Mavic, and given its low ground clearance I can't imagine anything that would work or that I would trust with my machine.
I would caution you to tread softly down this road, though, as the Mavic is a finely tuned engineered machine, and any alteration to its original design, i.e., floats, etc., may have seriously bad consequences. Something to always consider whenever you mess around with the Mavic's original thought-out design. And just so you know, using any Mavic accessories not made by DJI voids your DJI warranty, including landing gear, props, prop protectors, anything.
I also live in a water-based environment so I know what you are facing. However, I would advise you to simply work with your environment instead of forcing a square peg into a round hole. Electricity and water simply do not mix, and your Mavic is a mass of circuits, chips, and electric connections, signals and impulses. Water would be one of the Mavic's natural enemies. I can't think of a good reason to purposely put the two together that still fits in with good ol' common sense.