The speaker describes the sea as "calm and deep" and "full of light", and these images suggest that the sea is a source of peace and tranquility. The speaker also finds comfort in the sound of the waves breaking on the shore, which he describes as a "solemn sound". The speaker finds comfort in the fact that the sea is always there, and that it will continue to exist even after he is gone.
The sea is also a symbol of power and strength. The speaker notes that the sea can be "wild and stormy" and that it can "rise and fall", and these images suggest that the sea is a force to be reckoned with. However, the speaker also finds comfort in the fact that the sea is ultimately benevolent, and that it will eventually return to its calm and peaceful state.
The sea is a useful symbol to the comfort speaker in "Break, Break, Break" because it represents both the beauty and the power of nature, and the speaker finds solace in both of these aspects. The sea is also a symbol of eternity and continuity, which provides the speaker with a sense of comfort and hope in the face of grief.