This will make the fish you catch with the Sinking Lure be legal, as if you had caught them without it. After getting the Sinking Lure, go to the fisherman and say to him "Let's talk about something else". He will then allow you to use the Sinking Lure, and the fish you catch with it will no longer be noted as being "Illegal".
Note: You will still need to find the Sinking Lure each time you come back.
