This is a pretty simple achievement but can take a long time due to the fact you need a cannon rowboat, and at the same time need a kraken/megalodon to spawn.
When I did this achievement, I loaded into a server and looked around small islands and forts for a cannon rowboat. I recommend islands with a lot of beaches, such as Sandy Shallows or Old Salts Atoll. Also, cannon rowboats don't spawn at sea posts or outposts (but you may find one left over from another player).
After I got a cannon rowboat, I sailed around and played normally until a megalodon spawned on me. I raised the sails and put the boat into a turn so I was turning with the megalodon and shot at it 25 times until the achievement unlocked.
Hope this helps and good luck :)