Good question! Bay and gulf are very similar. A bay is part of the coast where the water comes inland. So it is a body of water almost surrounded by land on 3 sides and is usually connected to the ocean. It is usually shaped like a U or or a C.
A gulf is a very, very large bay like the Gulf of Mexico or the Gulf of Thailand.
Loch is the Gaelic word for lake. It is mostly used in Ireland and Scotland. Loch can also mean inlet, which is where the ocean comes into the land as a long, narrow body of water. It resembles a thin lake or river of ocean water, rather than the open U or C shape of a bay.
Port and harbor can be similar. A port is bigger and has more equipment because it is where large ships can dock and load or unload people and cargo.
A harbor is where ships can seek shelter from bad weather. Some harbors are built almost like a port, but others are not built. They can just be naturally safer places for ships to go, like a bay or inlet.