I don't feel comfortable going to the beach with my friends?

Sounds like you could be a real heartbreaker!;) Who cares what others might say or think? Youre beautiful and you should show it. Be proud of your height and your great body. Not many people are 5'11 and curvy. This time dont let yourself be pulled back. Dont keep on thinking "What if this or what if that". Your best friend is anorexic. So what? Being thin as a stick isnt attractive for the guys. They like girls who look healthy and who have meat on themselves(just like you). Just stop being afraid and be confident. This time dont allow yourself to make an excuse. Your buddies would love you to be there (since they ask you every single year). And if they are true friends, they wont judge or be critical about your beauty. If they are, they arent worth it. Remember to pack sunscreen, put on a nice bikini and go enjoy yourself!:)