In part, the nautical imagery is significant because it illustrates the setting of the tale told by the Pearl Poet. It is the often familiar and old tale of Jonah's rebellion against God's call to be His prophet, speaking His Word. Noah rejects the call and escapes to sea thinking God will never find him there in so vast a featureless space. God, of course, does find him and stirs up the sea against him. While the sailors and captain are trying to determine what has gone wrong and for what they are being punished, Jonah declares his guilt and find himself a victim of the sea's raging power. Then he is swallowed by the whale.
they seized him ... and heaved him overboard into the raging sea. [...] when [the whale] saw the man being thrown into the water it swam swiftly towards him with its great mouth open. (Richard Alan Scott-Robinson, Modern English Translation)
In another way, the nautical imagery is significant for the symbolism it conveys. A few of these nautical symbols are water, waves, and the whale. Briefly, the symbol of water represents infinity, or eternity; the symbol of the waves represents the might and power of God; the symbol of the whale represents separation from the "fine virtue of mercy" of God.
Water: Jonah thought that in such a vast space he would be safe from God's observance. In truth though, God proves that he is the master of infinity by finding Jonah and overpowering him right there in the vast space of featureless sea:
for [Jonah] knew ... that the person who had made the Earth and all the plants and animals ... had no power to trouble anyone on the ocean!
Waves: Waves plummet and pound until Jonah (with the sailors' help) realizes the right course to travel for the safety of all and is plunged into the watery vastness. God proves His might and power by the terrifying action of the waves, and Jonah is brought to his first moment of submission.
the vessel rolled ... on the mountainous waves; the sea struck from behind ... then it surged again, overwhelming the helmsman and engulfing the stern.
Whale: Often used as an analogy for Christ's death and entombment of three days and nights, the whale separates Jonah from God's "fine virtue of mercy." It is this separation that causes Jonah to desire God's companionship regardless of the arduous task to be done.
Jonah stood up ... [it] stank like the devil, it felt like being in hell, ... At last Jonah ... cried to Him: 'Prince, have pity on your prophet! ..., please stop this vengeance and display your fine virtue of mercy!...'