As mentioned in "Aeration & Deep Root Feeding", roots need oxygen.
But how do their roots get oxygen in wet areas such as swamps, coasts and tidal areas? Well, pretty much the same way you would if you were submerged in shallow water.
Pneumatophores are adventitious roots that rise above the water in order to access oxygen that is unavailable down below. Meanwhile the fine roots below can continue absorbing water and nutrients.