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.
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