<span>The adverbs listed are:
resolutely
reasonably
truly
fiercely
emphatically
</span>
The theme highlighted in the mentioned excerpt would be perseverance.
The excerpt tells of Rainsford's courageous act of braving the sea to go to the direction of the shots despite of the harsh conditions at the sea. This is evidenced by the phrase "he fought the sea".
The term "doggedly" also describes a persistent pursuit which was what Rainsford showed. He showed perseverance in his actions.
