The term 'shoreline opening' can be broken down into two crossword solutions: INLET (5 letters) and RIA (3 letters). An INLET is a narrow body of water that connects to a larger body of water, such as a bay or ocean. A RIA is a coastal INLET formed by the partial submergence of an unglaciated river valley. Both inlets and rias are important features of shorelines, providing access to the sea and creating sheltered areas for boats and other watercraft.