Sea
Use for 'ocean,' a much larger body of water than a sea.
Use for 'beach,' the sandy or pebbly area next to the sea.
Use for 'coast' or 'coastline,' the land along the edge of the sea.
The sea is blue and very big.
I like to swim in the sea in the summer.
Let's go to the beach to see the sea.
The boat is in the ocean.
In both Latin America and Spain, 'mar' is almost always a masculine noun: 'el mar'. You will see 'el mar azul' (the blue sea). It is correct and what you should use in conversation.
While 'el mar' is standard, you might hear sailors, fishermen, or people in coastal towns say 'la mar'. This is a poetic or traditional usage and is not typical for everyday conversation by learners.
Across Latin America (Mexico, Colombia, Argentina, etc.), 'el mar' is the standard and correct form. Using 'la mar' is very rare and sounds literary or old-fashioned.
Using the wrong gender. For example, saying 'la mar es bonita'. — Always use the masculine gender for everyday conversation: 'El mar es bonito'. While 'la mar' exists in poetic contexts, 'el mar' is the standard for learners.
Confusing 'mar' (sea) with 'playa' (beach). For example, saying 'Estoy en el mar' when you are on the sand. — Use 'mar' for the water itself and 'playa' for the sand next to it. Say 'Estoy en la playa' (I'm on the beach) or 'Estoy en el mar' (I'm in the water/sea).
Using 'mar' when you mean 'océano'. — 'Mar' is a sea (e.g., el mar Caribe), while 'océano' is an ocean (e.g., el océano Atlántico). Use 'océano' for the vast, major oceans of the world.