His body was recovered from the water in Skegness after a five-hour search by police and the coastguard.
It is believed Kacper died on Saturday (13 August) after getting into difficulty in the water.
Emergency services were called at around 6.15pm and his body was recovered at around 11.30pm.
Lincolnshire Police confirmed he was from Hereford, but have not officially named him.
The force said the incident was one of multiple call-outs relating to missing people on Saturday.
A minute's silence is to be held in tribute to Kacper, who played for his local u14 team at Hereford Pegasus Football Club.
In a tribute on Facebook, Kevin Bishop, from the club said: "He was an exceptional player for the team and had recently been selected for the Hereford Football Academy.
"Kacper joined the club at the tender age of five and quickly developed into an excellent player dominating games from midfield.
"Kacper was a lovely boy on and off the pitch, and a credit to his family and club."All at the club are devastated by the news and ask that the family, management and his team mates are given the time and space at this most difficult time."
The club will be holding a minutes silence and a collection for Kacper at its FA Vase game on Saturday 27 August.
Meanwhile, a fundraising page set up to support Kacper's family's wish of having him sent to Poland has already surpassed its target.
The page, which is set up by a family friend Justyna Lebiest, has already reached its goal of raising £5,000.
She wrote: "His family have decided to send him over to Poland, and allow him to peacefully rest there.
"Unfortunately, this involves a pretty high cost to cover transport and the funeral.
"Any donation will be directly sent to his mother, and will be greatly appreciated."