The plot is (I believe) that a boy is on vacation with his parents in Egypt. The book is set around the time of the flooding of the Aswan Dam.
The boy meets this other kid (while scuba diving?) and they become friends. The story emerges that he is actually descended from survivors of Atlantis. They have this giant space ship that is travelling very near lightspeed. The current occupants don't know how/can't stop the ship. They can however slow it down enough for a small craft to leave. In the past, a man (Ra-kehty?) left the ship to visit earth. He settled in ancient Egypt and became respected. He had with him a device that could make things lighter. The boy has a smaller version of the device with him. The people on the ship have learned how to slow down the ship enough so that a small craft can leave and return. He is looking for this ancestors tomb, because the device could cause problems. The flooding for the dam also means that the tomb will soon be underwater and out of reach. The time for him to be retrieved is also coming. He enlists the help of the kid to dive and search for the tomb.
Eventually, they do find it. It is sealed with a massive granite slab. The boy uses his device to make the slab move outwards enough that they can dive under the slab and enter the tomb. Inside is the man on a bed. The walls of his tomb contain his diary. The boy takes the man's device and also records the walls of the tomb. As well, he takes the bones of the mans right hand in a bag.
They leave the slab as it is and go. The kid reports the discovery of the tomb (with some details left out) and the boy returns to space. The end has a news clipping about the discovery.
Read during the 80s.