I take the opposite view of Sonja.
Its your name, you will be the one that has to live with it. It'll be the name you put on all of your legal paperwork, the name that's on your ID and the name that will go on your medical records. If you want to be named Serenity, then that's your choice.
However, if you don't have a problem with your family giving you a nickname like Serena, then again, that's your choice.
And for the record, I used to know someone named Serenity when I was a kid.
From someone that also has an unusual name,
Ryuichi