Why does Lessing portray the sea in "Through the Tunnel" as the real sea?
I am not too sure I understand what precisely it is you are asking. However, I will do my best to infer your meaning and if I don't respond to your specific question, please get back to me and I will see if I can help further. What is important to realise in this story is that the sea is presented as a dangerous entity, which is perfectly capable of claiming the lives of those who are not careful. Consider the description of the sea that we are given the first time when Jerry goes to the wild bay by himself:
He went out fast over the gleaming sand, over a middle region where rocks lay like discoloured monsters under the surface, and then he was in the real sea--a warm sea where irregular cold currents from the deep water shocked his limbs.
Note the way that the sea is portrayed as a very dangerous place. The simile describing the rocks like "discoloured monsters" makes this perfectly clear, as does the mention of the current that "shocked" Jerry's limbs. The reason why the sea is portrayed in such a realistic and dangerous manner is to emphasise the risk that Jerry takes as he tries to go "through the tunnel." This is no silly childish dare. This is something that could result in Jerry losing his life and in his death. The challenge of going through the tunnel has to mean something, which means there has to be a risk. If the sea were not so dangerous, it would greatly lessen the impact of Jerry's achievement.