al mar is spanish for to the sea.
al is actually a el. The two words join together to preserve the sound and so that it sound wear when saying it.
a= to
el = the
mar = sea
the sea
el océano/el mar
the sea
(el) mar
sea: el mar ocean: el océano
sea = el mar
In "The Old Man and the Sea", there is no difference in meaning between "la mar" and "el mar". Both mean "the sea" in Spanish. Hemingway may have used different articles to add variety to his writing.
Bajo el mar.
el mar-spanish for the sea
El amargo del mar
el dolfin vive en el mar
Sea
The ocean in Spanish, according to Google translator.