Born as a water elemental, Besmara was always ambitious. Over years, she found she had a talent for bullying sea monsters into doing what she wanted.
She became a powerful water spirit worshipped and channeled by the shaman of Garund, essentially running a protection racket where sailors would offer her gold so they wouldn’t have their ships hounded by sea monsters.
Long story short, she was eventually able to steal a fragment of divinity. The appearance she is mostly depicted with is dictated by the population of the Shackles, the country where she is most widely worshipped. Her more shamanic roots are known, but everyone knows you start fresh when you move to the Shackles.