Unlike Hikkaduwa, Tuna isnt a surfers beach though it is possible, but there are hidden rocks. Most surfers gather at the West end of the bay, it would be wise to consult other surfers before venturing out.
Beach sellers with all the paraphrenelia prowl around, but avoid them and they easily move on. After a few hours, they leave you alone to prey on more recent arrivals. Some of them are good value, others aren't. Some locals might offer you drugs. I have no interest in drug-use and would advice you to say no.
Marlboro, 555, Camel and your fav brand will be available, but they arent authentic. If you ask any of the beach restro owners, they will say its genuine to promote sales, but avoid buying fags on the beach.
Unawatuna is a gorgeous beach, rightly deserving the discovery channel's title "best beach in the world" but...it does have a seedy side. Like a lot of beaches in asia that are popular with tourists, over the years a drug problem has developed and in Una it's heroin. Just take a bit of a longer look at some of the men offering to sell you coconut on the beach and you'll see that many of them are incrediably thin (not through starvation) and have track marks on their limbs. It's up to you if you want to pay for their next fix, but there is a nice elderly plump lady who also sells coconuts and pineapples, who i felt a lot happier handing my money over to.