Hey there. I've been using the TSP for quite a while now. If you go to the website
www.tsp.gov you can take a look at the historic returns. Personally for someone who doesn't have a whole lot of time to research different stocks and such, I think the TSP is an excellent plan.
The reason why the TSP is pushed hard towards junior enlisted is because it's never too early to start saving for your future. Look at it this way, you are taking out 10% pretax from your salary and putting it towards your future. It will grow. It is better than putting it in the bank and earning very little interest. If you decide to put in your 20 or serve out your enlistment, you will be a step ahead already.
10% is a good start, and of course if you can afford to put more in, do it! If not, it's not that bad. I don't know how the Navy's deployment schedule is, but if you get deployed, use that as an excuse to contribute a little bit more!
Personally I like to put my money in a little more risky (but higher possible return) funds. I currently have my distribution as 30% C, 40% I and 40% S. I'm basically basing this off of how the market has done in the last couple years. Putting it all in C is also a safe bet. I would stay out of the F and G fund for now, as that's mainly for those who want a steady return but little risky - i.e. someone close to retirement.
I'm not sure what the L fund is. Is that a new addition? PM if you have any specific questions.