As for your question, ewww… I’m sorry but I think you might be out of luck sadly, as the Underwater materia is one-of-a-kind and can only be gained from the Kalm Traveller. However, because the item used to gain it is re-attainable (a la, the Guidebook by morphing a Ghost Ship in the Junon reactor tunnel), you might be able to regain another Underwater materia by giving him another guidebook.
I haven’t tried that so that’s a big assumption on my part though I’m sorry.
But, honestly, the materia isn’t NEEDED. The fight it’s used for is 100% doable in the time limit imposed without the materia