Galveston Beach Patrol said a woman with five children went into the water Saturday afternoon and they stepped into hole where a pier used to be near 21st at Seawall.
The lifeguard was trying to move them. Three of the children -- ages 12, 13 and 13 -- got out of the hole by themselves.
After the two children were out of the hole, the woman was left holding the other two children above water -- a 12-year-old girl and a 9-year-old boy.
A wave hit them and she lost hold of both children, officials said.
The lifeguard saved the boy and the woman was able to free herself, but the girl was still under water.
Police said the girl drifted away. The lifeguard found her after 23 minutes and performed CPR.
She was taken to the University of Texas Medical Branch in Galveston breathing, but she died Saturday night from her inuries.