Sealnado scatters items it collects around the area and Lucy is buoyant, so she's probably not at the bottom of the ocean. I once lost 2 Cutlass Supremes while fighting it and found them in the ocean around the island later, completely unaffected with the same durability as before.
Your best bet is to sail around the shallow ocean boundries of the island the fight took place on or near, then scan the island itself. If you don't find her, scan the medium oceans. If still not, then that's all you can do, and your only hope is using the Console.