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Girls
Amaya (a-MYE-uh ... "rain" meaning, so this is maybe pushing the water theme a little, but still)
Marina
Maris
Marissa
Meriel / Muriel
Merlyn
Michal (pronounced like "Michael")
Rhea

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Boys
Channing
Conway
Merlin / Merlyn

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Either
Aeron (pronounced EYE-run ... the name of a river in Wales as well as a Welsh name)
Lake
Morgan (If you want to be technical, Morgan as a female name probably has a different origin, but as a male name it's water-related in meaning)
Ocean
Rain