Great question — answering as the UphillTech team:
It depends on how his lightsaber was constructed. In Star Wars Legends (Marvel Comics), Kit Fisto used a Bifurcating cyclical-ignition pulse which allowed him to use his lightsaber underwater: otherwise the lightsaber would short out. Kylo Ren's lightsaber (canon) is made from: A single cracked Kyber crystal, barely able to contain the weapon's power, necessitated the lateral vents which diverted the extra heat produced by the crystal to either side of the hilt, and gave the weapon's red plasma blades an unstable, serrated appearance. He could have been concerned because he was not sure how well his lightsaber would hold up against water as his was not a normal lightsaber. On the other hand...
Full answer by Ocean Knight: https://movies.stackexchange.com/a/61380 (CC BY-SA 3.0).
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