The sea here is a symbol for the unconscious, but also for unconscious desires which may be deep and dark in their very nature.
The sea is something that can help Aschenbach to explore his own desires whilst he is in Venice, something which has actually been the source of a huge amount of emotional turmoil for the man since falling in lust with a teenage boy.
They are both about a forbidden love, and there are elements of pedophilia in both, although the term for the type in “Death in Venice” is pederasty.
Both stories are rich in contextual symbolism, ideas which only make sense within the context of the book.
In “Death in Venice”, Thomas Mann uses a number of these symbols to illustrate points that he was making.
The topic of the story is homosexual in nature, which also means that Mann may have had additional pressure to hide some of the ideas he wanted to use behind the cloak of symbolism and imagery.
As a result, the pederasty of the book is a contextual symbol for something larger, the ability of the human race to idealize things that it thinks are wonderful at the peril of forgetting the things that are not.
Ancient Greece is seen as a bastion of hope, democracy, freedom and intelligence, but there were also elements of their culture that would certainly not fit in with the moral fortitudes we make today.
It is noted that Aschenbach has “the feeling that the eros which had taken possession of him was in a way singularly appropriate and suited to such a life”.
It is clear here that his feelings towards Tadzio are being used as a symbol for everything that was wrong about Greek culture, to explore the fact that our romanticizing of Ancient Greece may be reducing the role of some sinister elements of that culture.