I would think you need to drop it to clean it out.
Pull sending unit out, and line, go to a floor drain. Start filling it up with a hose, you need to flush out all of the sugar. And if anyone asks what that is, tell them a WATER TANK from your boat, if you say fuel tank and EPA finds out you are fucked.
After all the sugar is out (best guess), try to get all the water out you can, and then let it sit and dry.