Eluned was wearing a lifejacket but it wasn't the appropriate type for the heavy sea conditions that day, according to a coroner's report of her death.
Her father, Geoff Finney, said he had purchased a red compliant lifejacket for his daughter, which she was wearing when the boat left the shore.
Without his noticing, she exchanged the lifejacket for another, which had a buoyant collar but was not designed to keep the head and face of an unconscious person above the water.
Eluned was found face down in the water wearing a brown camouflage lifejacket.
It might have been "prudent" for Mr Finney to have arranged the boating trip accompanied by another boat, the coroner said.
"Geoff Finney, by his own acceptance, misjudged, and therefore was caught out by the sea conditions he encountered.
"His skills may have allowed him to negotiate the choppy waves to gain a place of safety but the failure of the engine precluded this with tragic consequences."
Mr Finney said he regretted not having a GPS distress beacon with him, however it was likely Eluned still would have drowned before rescue services responded to a distress signal, the coroner found.
The boat and its motor were not recovered so it was not able to be discerned why it had failed.
