Rihanna turned 25 years old yesterday ... and she celebrated by throwing on one of her smallest bikinis and hitting a jet ski in Hawaii.   TMZ.com RiRi and Brown spent the whole day together ... holding hands, jetting around in the ocean ... and probably smoking weed.There were rumors that Brown was planning to pop the question during their Hawaii vacay ... but there doesn't appear to be a ring on her finger ... yet.Read more:   [fblike]