Princess Gwenevere and the Jewel Riders is a 1990s television cartoon series inspired by the Arthurian myths and starring Princess Gwenevere, or Starla as she is known in some distribution circles. Gwenevere and her friends Fallon and Tamara are the central trio, the three of them owners of magical jewels that give them powers and armour. They use their jewels to fight evil and protect the helpless in the kingdom of Avalon.
In one episode, "The Jewel of the Sea", Gwenevere and her friends travel out of Avalon by ship, in search of an enchanted jewel that turns out to be underwater. On this adventure, the group encounter a merboy named Gilly and an unfriendly sea monster, and in the process temporarily become mermaids themselves.
Thank you to Char for letting me know about this episode!