The 16-year-old drowned at Hillarys Boat Harbour after attempting to fetch a rugby ball in the water near a jetty about 4pm.
Attempts to resuscitate the teenager at the scene were unsuccessful and he was pronounced dead at Joondalup Health Campus.
The Beechboro resident was the seventh born in a family of nine children, who arrived in Australia from West Africa at the turn of the decade.
His younger sister Yamah described the grief the family had endured this week. “It’s really hard for everybody,” she said.
“Still can’t believe it, I guess we just have to be strong.
“He was a nice, caring and loving brother who loved sport and always put people first.”
Yamah’s thoughts on her older sibling were echoed in many social media tributes paid by friends to the teenager.
Facebook user Chayne Hetutu said his friend was “never once… sad or down” and “kept everyone around happy”.
Moesha Teiri wrote that her friend was “always so happy and always had a smile on his face”.
Olie Niyonsaba posted his disbelief.
“No! No, this aint real, few nights ago we were talkin’…u made me believe in myself (sic).”
Bayswater Football Club players, based at Hillcrest Reserve, will wear black armbands in Amos’ memory on Saturday.
A Go Fund Me page has been launched to assist the family with funeral costs at www.gofundme.com/amossumo1.