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
There will be a three-way split across the UK this afternoon, weather-wise.
Nigel Farage has been forced again to field questions about his party's members on the day he wanted to be discussing a new election pledge.
Following Bradford City downing Premier League leaders Chelsea on Saturday, we take a look at the greatest FA Cup upsets.