Answer:
The body of water that is most likely to be a marine ecosystem would be an estuary.
Step-by-step explanation:
An estuary is a widened, often funnel-shaped mouth of a river, where fresh river water and salt sea water are mixed and thus brackish water is created, and where tidal differences can be observed. When a river flows as a system of branches, it is called a delta.
Estuaries often have a great natural value. They typically consist of swallows and salt marshes that are very rich in invertebrates and thus have a great attraction for birds, among other things.