You have to get thescuba gear from the picture guy. Then you go to the light house and dive into the water. Get the crab trap and the light house keeper will give you a lobster and the keys.
Even if you have the jetski key and know where the map is (under the rock on Seal-Saw Island), you still must look through the telecope at the lighthouse, because that's how you are supposed to find out where it is. (see related question)
The lockers on Big Nate Island have combination locks. Nate's is written on the comic pieces.
The key fits the jet ski, so you can race Nate out to the seal island to the right of the lighthouse (you can see it through the telescope). The map to the time capsule location is there. (see related question)
keep trying
i belive it is a key or locker key
Spitsy the lost dog doesn't want to leave the playground. In fact, he is the key to uncovering the Time Capsule, since you don't have a shovel. Go to the Kids Only room with Nate and play "Go to Jail" (Hangman) to get some peanut butter crackers for Spitsy. Once you have the map from the jetski race, and ring the bell to get the girls away, Spitsy will dig up the Time Capsule. (see related questions)
Spitsy the lost dog is wandering on the Playground on Big Nate island. He is the key to recovering the Time Capsule because he will dig holes to save any treats he gets. Once the girls are off the playground, and you have the map from under the seal island rock, you can use the peanut-butter crackers to encourage Spitsy to dig up the Time Capsule. (see related questions)
The only thing in the "secret room" under the detention room is the missing School Bell Clapper, which can repair the bell once you have scared off the seagull.
Beat The Diving thing and talk to captin salty in big nate.
I think it belongs to Britain.
There are keys on several islands.Shark Tooth Island : the "translation key" is a page from Professor Hammerhead's Journal. The "key ingredient" is for the shark potion.Big Nate Island : if you recover his lost lobster trap, Cap'n Salty will give you the key to his jet ski.Astro Knights Island : the small key opens Mordred's underground lab. The Golden Key opens the path to Mordred's hideout with the portal on the crystal asteroid.Counterfeit island : the key opens the Inspector's house in the Country Side.
The diving mini-game is the way to get this. You will need an old photograph that is blowing in the wind near the lighthouse : swap it for scuba gear at the "Say Cheese" photo shop. Then dive for the old sailor's lost lobster trap. He will let you keep a lobster and give you the key to the jet ski. Use the telescope on the lighthouse, then race Nate on the jet skis, going out to the seals' island for the map there.