Actually "sea" is a countable and an uncountable noun (according to the Cambridge Dictionary). The definition of a sea is almost the same as the defintion for an "ocean" (i.e. salty water / not fresh water). So, in English we have an expression that uses the word "seas", e.g. I have sailed (travelled on a boat) on the seven seas (7 oceans). If you live beside a sea (one of the many seas in the world) then you can say that you live beside the sea. I don't know which sea you live beside, so I would have to ask you "which sea do you live beside?" a sea = one sea (any sea) the sea = means I know the sea that are you referring to (talking about).

"The sea". "A sea" does not define which sea, and if you live seaside, it's doubtful you have the option of several seas... only one: the sea.

I live near the sea