YA book about teen Atlantian in Egypt

Question

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.

Answer 1

King Tut's Game Board by Leona Ellerby

While vacationing in Cairo, Justin Sanders meets with Nathan Alistant and the two boys explore the pyramids, Tutankhamon's tomb, and the Temple of Abu Simbel.

Reviewer on amazon mentions it being blend of science (underwater archaeology) and fantasy (Atlantis).

It can be read on archive.org, some excerpts from page 88 where they find man's skeleton:

Justin drew in his breath when he saw what was there - a skeleton crouched in the corner, hands folded, a golden necklace around its neck. "Uncle Aye," Nate whispered.

... Nate was busy too. First of all, he went over to the skeleton and carefully removed the index finger of the right hand, wrapped it , and put it in a small waterproof sack.

...Nate seemed reluctant to leave the skeleton, but finally he went to the center of the room and very carefully opened the lid of the alabaster chest. Inside were two rolls of papyrus and a device just like the one he had used to move the rocks. Nate took out another waterproof bag and placed the strange device and the papyrus rolls into it.