"Our father swims in the sea."
Translation:Nuestro padre nada en el mar.
Yes, in more poetic contexts or florid phrases, the sea is treated as feminine. Whenever we use an archaic maritime phrase in English, I am alert to the possibility of the sea being feminine in Spanish. For example, "the high seas" is translated as "la alta mar" and the Spanish for "to set sail" is "hacerse a la mar".
Whenever you use sea to name a specific body of water, it is always masculine, e.g. el mar Egeo or el mar Báltico.
Interestingly, when one is describing unfavourable conditions, the sea is suddenly treated as feminine – a choppy sea is "la mar rizada", rough sea is "la mar gruesa". To fix this concept in my mind, I imagine an old sailor with weather-beaten features, puffing on his pipe whist staring towards a Spanish ship on the horizon and saying in his pirate-like voice: "aye, the sea can be a cruel mistress". Seems to work for me and my strange way of thinking. I hope some of this was worth the two year wait...
Yes. The possessive pronoun acts like an adjective and matches number (and gender in nuestra/nuestro/nuestras/nuestros, vuestra/vuestro/vuestras/vuestros).
This one felt a little counter-intuitive since it means that your several poor brothers' shared house uses "su casa" and your one rich brother's many houses uses "sus casas".