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
A British teenager arrested in connection with cyber-crime offences is suspected of hacking an email account used by the head of the CIA.
A man who hit a 13-year-old boy leaving him bedbound for life has been jailed for three years.
Migrants as young as 10 have been attacked by French police and armed civilian "militias" in Calais in recent weeks, a charity has claimed.