Lily Sullivan’s body was found in Mill Pond, Pembroke, in the early hours of Friday, 17 December.
Lewis Haines, 31, of Flemish Court, Lamphey, Pembrokeshire, is due to stand trial for murder on 20 June.
At a hearing at Swansea Crown Court on Monday, Haines pleaded guilty to a lesser charge of manslaughter but the trial on the charge of murder will go ahead as planned.
The trial is due to last for two weeks.