your plug isn't leaking. It is there to release any water that has gathered in the lowest part of the lazerette, often from your outboard motor.
Best not to remove it when the boat is in the water as it has a very fine plastic thread and it is very difficult to screw the plug back in correctly.
When next checking the lazerette taste the water and establish if it rain water or salt water.
Then you can work backwards and find the cause.