I've used my MXT at beaches in Newort and Middletown, RI. Lots of iron because beach houses were destroyed during the hurricane of '38. Although the 9.5 and DD coils are waterproof, when a wave is coming in, I raise the coil because immersion causes very harsh noises similar to those made when detecting too close over a large and shallow target. Wait for the water to receed, then keep swinging. Last year I bought a Coilsearch coil from Dixie Metal Detectors for my Sov. Elite. This coil was one of the 150 coils Dixie purchased from Minelab because Minelab found a bunch in their inventory that hadn't been sold. I asked if this coil was waterproof and was advised to run a bead of silicone sealant round the coil edges. Since I have a DD coil with my MXT, I also ran a silicone bead around the edges. Although this coil might be water proof, this simple procedure gives me a little more security and the sealant won't seep into the coil. Don't use spray sealant - it'll wash off. You just want to seal the edges.
mike