Can I assume your boat is aluminum as has rivets?? If so the most likely culprit is loose rivets. Not uncommon is this type of boat.
The reason it's not leaking when your in it is because you have increased the pressure on the hull and the aluminum cross members thereby sealing it better. When it's just sitting, there is no inward pressure on the hull and water is getting in around a couple loose rivets as the hull expands and contracts with heat. Aluminum has a LOT of thermal movement.
For a temporary and quick fix, seal over the rivets with a high quality silicone from the outside. You don't need much. For a permanent repair, replace the loose rivets.
Good Luck!
P.S., if your boat is not aluminum disregard everything above!
Dan