Ambiguous Nouns

Ambiguous Nouns
 

Is it correct that these nouns below can be either masculine or feminine?

el azúcar / la azúcar
• Hay mucho azúcar en el estante.
• Hay mucha azúcar en el estante.

el mar / la mar
• Me gusta nadar en el mar.
• Me gusta nadar en la mar.

If so, are there any more nouns like these?

Yes, there are others, like "sartén", "reúma", "linde", "calor" (although this one became accepted in the feminine form because a lot of people use it, but for educated speakers it sounds incorrect), "tilde" (this one is much more often used in feminine).