Boat Easter egg
From Halopedia, the Halo wiki
In Halo 3, on the level The Storm, there is a large boat on the lake bed in the area where the player has to destroy the Scarab and AA Wraiths. The boat is beached on the bottom of the lake bed due to the Covenant's excavation of the Ark's Portal. The digging has caused the lake's water to drain, leaving the boat stuck.
To get in the ship, the player needs to be in Co-Op. Have one player in a Ghost. Then go up the ramp the player came from. Look to the left, and there will be a lamp. To the right of the lamp, there is a gap. Position the Ghost so that it can boost out the hole towards the ship. The player must jump on the Ghost and get their partner to boost out the hole. About halfway to the ship, they must jump off the Ghost. If they jumped at the right time, the Ghost's speed should have kept them going, so they land on the ship. There is an easier way to see this Easter Egg, one can just need to go to the theater, press Y so they can move freely and get into the boat with the camera. And as another way, one can use the Flying dumpster glitch, which may lead them to the ship.