The situation here has three important factors that jump out at me:
- You want to do something that neither of the others do (or, rather, you don't know if they want to do it). I've been through this sort of thing, especially when hanging out in groups.
- You're looking down your nose at your friends a bit for what they're doing. You think it's a bit stupid - "immature" might be a better word.
- You do care about your friends. You're making your choice in part because you know they're having fun. Good for you!
I'm shy, and I've had to deal with the first problem a lot. For instance, at birthday parties as a kid, I sometimes found myself wanting to do something that was different from what the majority of the group wanted - to take a specific case, playing ping pong instead of soccer. I didn't hate soccer, but I wasn't good at it (and everyone else was), and so I had a hard time being as enthusiastic about it.
Try to be a leader.
I've dealt with that by taking the lead more in these situations. One thing I've realized is that sometimes you need to be a leader. At one of those birthday parties, for example, it turned out that other people in that group did indeed like ping pong - but they were going along with a sort of herd mentality. Had I spoken up earlier, I could have turned the activities in a different direction that most people still would have enjoyed.
In your case, yeah, your friends are having fun doing one thing. But you don't know for sure that they don't want to do another. You've got nothing to lose by simply asking, the next time around. This has the advantage that it can end up pleasing everyone, if you pick the right activity.
There are some different approaches you could take when you phrase things. You could ask right off the bat:
Hey, do you guys want to play tag? I think the sand would make it a bit more exciting.
or you could offer some sort of compromise:
I know the ocean's great, but can we split our time between the water and the beach? I was thinking we could play tag.
Here, you should be assertive, but not so much as to come across as bossy, by accident.
To be honest, I think the second of these might be the way to go. As I said, you care about your friends, and you're not being selfish. This option should make everyone happy.
I would, however, recommend that you ask to play tag first, if possible. Honestly, once kids get into something, it's hard to tear them away from it. You do run the risk of all getting absorbed in one activity, and then you don't have time for the other. If you think you can hold yourselves to a schedule, then by all means, do so.
Gender might very well be a factor here. Both of your friends are female, and so it's possible that that has a bearing on their actions. I have no wish to perpetuate gender stereotypes, but as adolescents, children often socialize more with members of the same gender. This isn't true all of the time, but it's often the case.
American culture does still carry the idea that girls should do "girlish" things and boys should do "boyish" things. I have no comment about whether or not tag is a "girlish" or "boyish" activity; in my experience, it's neither. However, those expectations often influence things like the books, movies and music that children and teens consume. I suspect that gender could be a factor behind the divide, and the reason they seemingly relate to one another better than they relate to you.