Oh oh, pick me, pick me!!!
Well, let's see, 2-3rd level PC's? How about what I've actually seen on the beach first.
A crapload of crabs. I'm not kidding. Picture 5-600 hand sized crabs. Doesn't sound scary? Well, they ate a dead seal in about 20 minutes. They could probably finish a scrawny mage off in about 3.
A ship, sunken so that it's masts and upper decks are clear at low tide. (No kidding, it's right out there)
Logs, lots of them, scattered on the beach. An undersea earthquake shook a bunch of logs off the bottom. Walking was dangerous, and the sneaker waves would roll the logs. More than one person got crushed by them.
Wine bottles. (No kidding. Crates and crates of them)
A chase in the dark, and a dead whale on the beach. The PC's are riding horses, and slam right into this dead humpbacked whale. Trust me, it hurts in a Chevette at 35!!!
Now, let's go with fun stuff for adventurer's.
A bottle with a note sticking out. The note reads: "I'm right behind you!" When the PC's turn around, have 3 or four goblins. Let the goblins have surprise, that will now be a serious threat to 2-4th level PC's. (As a side-note, I did that once. Put a note in a bottle, then hid in the dune-grass. When this guy and his girlfriend read the note, I ran down and stood behind them. When they stiffened up, I screamed "BOO!" then ran for it before they could beat the hell out of me!)
A mad hermit. Crazy as hell, who insists that the party is in league with the big otter that is stealing his oysters and making fun of him.
Oyster divers. Fun natives. And by the way, trust me on this, oyster goop or juice in the eyes, and you'll be damn lucky not to be blind. Plus, it gives you a horrible rash.
The kids from Lord of the Flies.
Ever seen that video of the killer whale whomping up on the ice and chowing down on that walrus? Think of the shock to adventurer's seeing that?
Sharks. They hit in less than 2' of water. No kidding.
Seagulls will defend thier nesting territory ferociously.
Those annoying brown bears we have around here. OK, they are more than annoying, they will chase you a good 2 blocks or until you spill someone's trashcan in front of them.
The guys off Deliverance. (I think some of those guys are my nieghbors)
A storm. EVERY winter we have at least two storms with driving rain, lightning, and the everpopular 100MPH wind. It sucks.
Riptides. Trust me guys, that riptide where I live grabs you, and you won't come to the surface until you are about 200 miles offshore.
Wrecked ships. We have a couple around here. Trust me, they are more dangerous than exciting.
That's just off the top of my head.
Oh, and if those kids can get away with paintballing tourists for over a month before the cops caught them, kobolds with blowguns can hide in the dune-grass and make the PC's lives miserable.
See, you have the beach, a good 50-250 foot stretch of sand, then you have the berm. That's a natural, 10-40 foot high dune, with grass on the top, then rolling, grass covered dunes behind, then the hill about a 1/4 mile back. The hill will have trees. All kinds of havoc is possible.
Oh ya, and seagulls in a crab feeding frenzy WILL attack humans.