The verb 'sounds' has an obscure and archaic meaning, which still exists in the nautical term 'soundings'.
The 'plummet' is a noun - not a verb, as more usually. It is a piece of lead (Pb - plumbum). The plummet is at the end of a string and you throw ('cast') it over the side of a ship to take a sounding (='check how deep the water is').When a poor man finds a long-hidden quarter-dollar that has slipped through a rip into his vest lining, he sounds the pleasure of life with a deeper plummet than any millionaire can hope to cast.
'The pleasure of life' in the quote is the water. I'm sure you can work out the rest of the image.
Student or Learner