"It was down a section of the beach where no one really walks," Gary Williams said. "It smells too bad, though. It's a very distinctive smell, like a cross between squid and farmyard manure."
Ambergris can sell for high prices to perfumers, who use the rare substance to make scent last longer.
The Williams' lump has an estimated value of more than $70,000 and has drawn interest from buyers in France and New Zealand.
"If it is worth a lot of money, it will go a long way towards buying us a static caravan. It would be a dream come true," Williams said.
Ambergris is not traditionally harvested from dead whales. Its sale is banned in countries such as the United States and Australia due to laws against exploiting whales.