Ambiguous Nouns

[fglorca]

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?

[AngelicaDeAlquezar]

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).