Since you'll probably be dead if and when it's found, you can put whatever you feel like on there. For example, some people really, really love the ocean. If you don't think skinny dipping is enough to prove that love, you can put some intimate — yet classy — jpegs on the 4GB flash drive and really commit your body to the deep. Just an idea.
And if a sailor comes across it a bit too soon? Well, all press is good press.