A 40-year-old woman, from Galashiels, Scotland, has been arrested on suspicion of murder and child neglect and remains in police custody.
Detective Inspector Nick Connaughton from Blackpool CID said: "This is a tragic situation where a young girl has lost her fight for life following an incident at a swimming pool in a hotel in St Annes.
"We are now treating this sad death as suspicious and are investigating just how the child has come into difficulties in the pool."
A spokesman for the 100-bedroom Dalmeny Hotel declined to comment.
More top news
Temperatures will reach around 28 Celsius (82F) at their highest.
A minute's silence will be observed at 11am on Thursday in remembrance of those who lost their lives in the Manchester Arena attack.
New research has found those who consumed two cups had a 35% reduced risk, and for those who drank five cups, the risk halved.