It must be so hard for you being so sporty and not being able to do them.
Dont worry though, until it gets better there is so much more you can do with your friends.
I wasnt the outgoing type when I was your age, I liked the beach too,but not very much into sports.
I like paper crafts and art type hobbies and I could recommend some for you to try.
It can be something you can do with your friends.
Take them along with you to the beach and plan craft projects which everyone can do and you can lead them (or if you have a friend who is good at crafts could help you learn).
You could all collect different things(sand dollars,drift wood,shells,sand etc )from the beach and create beautiful things,which you could use to decorate rooms,or gift to people on occasions.
Easy Crafts - Fun Projects and Activity Ideas for Children: Beach Crafts - Fun and creative ideas using the Beach Theme
Gift ideas: unique, homemade beach souvineers.
Hope you will have a look at the links and find something for you to enjoy with your friends.