Yes, the leaves need to be in the air. They can take being submerged for a few hours to a few days but this cant be a permanent situation. There are soo many ways to accomplish this, but David's link is an excellent resource to browse. My personal favorite way was to stick a length of rigid airline tubing into the sand, and then rubber band my mangrove in place in two spots so that it wouldn't slide off and would stay at a good level with the water line.
>Sarah