avoid sharp edges.. but #7 is too not serious... see my comment at the bottom for all of them... a sense of flowing is good.. try to tap into the idea of serenity.
Please note that you can use color in the logo, but it must also look good in black and white. (note there are 2 uses for the logo... on a website / print material and also on the actual device- which looks like a cell phone batter and has a grey background)
I would avoid sharp edges on the letters - because that seems to convey the opposite of the idea of serenity. Think of peace and calm when you are forming the letters.
It also would be nice if the font conveys a sense of easy flowing... imagine the soft sounds of the ocean playing in the background as the sun as setting as people gaze out into the ocean.