He was reportedly with a female friend on Sunday when the incident is thought to have taken place. Their sea scooter has been recovered by police.
Friends have taken to social media to pay tribute to Castro, whose real name is Theophilus Tagoe, despite his death not being officially confirmed.
Tributes have been tweeted by Lethal Bizzle, Sway and Tinchy Stryder.
Castro is currently number one in DJ Edu's 1Xtra Destination Africa chart with his remix of Sarkodie's Adonai.
Lethal Bizzle wrote: "My favourite Ghanaian artist :(. #RIPCastro".
Tinchy Styder tweeted: "My thoughts go out to Castro, his friends & family! Words can't even explain the shock I got when I heard the news! You're with the angels!"
His manager DJ Amess said that Castro had only returned to Ghana on Thursday and was celebrating with his friends, including footballer Asamoah Gyan.
He said Castro was joined by a woman but said it was not his girlfriend.
"She jumped into the boat to cruise with Castro," he said. "Castro refused her but the girl said that she would go.
"When they took off, the bystanders were calling for them to return so that the girl would put on her life jacket. Castro was already wearing his."
He said they did not hear calls from people who were standing watching him on a bank in Ada.
Footballer Asamoah Gyan expressed his sadnessWitnesses told him they saw Castro take off his life jacket to try to save the woman after she jumped into the water.
"After they were a few miles away, they saw the girl jump from the boat into the river," he said. "Maybe she was afraid.
"I wasn't there so I can't confirm it but somebody told me this. Then Castro followed suit. He thought he could have saved the girl.
"He was wearing the life jacket so the moment he fell into the water he was not able to save the girl.
"They told me they saw him removing the jacket. I wasn't there so I can't confirm whether it's true or not."
He also said he had spoken to Castro's parents.
"It's a disaster for the family," he said.