Firstly, I'd check the fuel take-off hose. This may be fractured or chaffed?
If ok, try checking the void when the boat has been ashore for a few days with a full tank of fuel. If there's leaked fuel present, it's likely to be a failure of the tank, as the sender unit, breather and fill hoses should be above the fuel level.
If no leaks, your problem is elsewhere. Sometimes the flange for the fuel gauge sender (if fitted) is prone to leaking. This would be the next place I'd go looking. After this, check the fill and breather hoses. A method I've used in the past is to plug the take-off hose and lightly pump some pressure into the tank through the breather hose, using a bicycle pump, toob pump, or similar. You should be able to hear any escaping air when the tank is under moderately light pressure.
If none of the above identifies the source of the leak, someone's pouring fuel in your bilge when you're not looking.