Firth and Lagoon

Firth noun - A part of a body of water that extends beyond the general shoreline.

Lagoon and firth are semantically related in nature topic. In some cases you can use "Lagoon" instead a noun "Firth", when it comes to topics like inlet, bay, body of water, shoreline indentation.

Lagoon noun - A body of water cut off from a larger body by a reef of sand or coral.

Firth and lagoon are semantically related in bay topic. You can use "Firth" instead a noun "Lagoon", if it concerns topics such as body of water.