Mizu Zokusei no Mahoutsukai @comic Gaiden: Penelopeia no Namida

Ryou, a water attribute mage, plans a comfort trip for his favorite sweets... no, to heal his friend Abel. The destination he chooses is Candia Island, where a grand festival is being held. While enjoying the warm welcome from the cheerful villagers, Ryou and his companions learn that the festival is actually a memorial service honoring a mage and an elf who fought and sacrificed their lives for the island. However, before they can reflect on this history, suddenly ferocious magical beasts attack the village! Ryou faces them to protect everyone, but... for some reason, he can't use his specialty water magic!? A comfort trip turns into a desperate situation on an isolated island with no help!? The side story of the strongest water mage's carefree adventure begins here!

(Source: TO Books, translated)