It's not difficult to seal and external screw or even a through bolt that has it head on the outside. From the OP's description it appears that the point of the screw has come through from inside the rig. Sealing the pointed tip of a screw is very difficult.
I'd want whatever was attached to the inside wall removed, a plug installed for a replacement fastener of the correct length, then a waterproof patch over the plug and the item re-installed, with additional sealant.
Then worry about closing the hole in the outside wall and getting the appearance to your satisfaction.
