General terms:
* Shoal: A shallow place in a body of water.
* Sandbar: A ridge of sand or gravel exposed at low tide.
* Flats: A shallow area of water that is exposed at low tide.
* Tidal flat: A flat area of land that is alternately covered and uncovered by tidal water.
* Shallow: (adjective) Describing the water itself.
* Wading pool: A shallow area of water that is safe for wading.
More specific terms:
* Reef: A submerged ridge of rock or coral.
* Mudflat: A flat area of land covered with mud, often exposed at low tide.
* Estuary: A place where a river meets the sea, often with shallow water.
* Lagoon: A shallow body of water separated from a larger body of water by a barrier.
* Backwater: A shallow, slow-moving section of a river or other body of water.
The best word to use will depend on the specific context of your writing.