In fact, Tagore may not have had many ideas at the time, and he may have written it as soon as he thought about it. It may mean that some eternal questions, such as those related to morality and conscience, do not require any answers. Silence means that such questions must be thought about by oneself, because everyone's answer is different.
Sea water and sky are just metaphors, because these two things seem to be easier to talk about and more "eternal"!