Hallie

Spelling variant and transferred use of the surname Halley. According to the Social Security Administration's name statistics, Hallie was used as a male given name in the US from the 1880s through the 1910s.

I think this is a soft and pretty nickname for Harriet, a name I have never cared for - in fact I find Harriet lacking in beauty and charm although it's popular among the posh set in the UK.
Hallie has been used as a nickname for Harriet since Regency times and perhaps earlier - Hal is a short form of Harry and the Hallie nickname reflects this as well as the usage of Dolly for Dorothy, Lolly for Laura, Molly/Polly for Mary.
It was even used as a Harriet substitute in the late nineteenth century, once Harriet had filtered from the upper and middle into the lower classes.

It means "thinking of the sea." Possibly derived from "hallelujah" (alleluia, etc).

Means "thinking of the sea".

Hallie and Hayley are 2 totally separate names. I like Hallie better anyways.

I don't think that Hallie is primarily a variation of Hayley. Hallie was in frequent use as a given name in the 19th century long before there were any great number of girls named Hayley. Hallie was originally a pet form of Harriet, just like Hal was a pet form of Harry or Henry. It's only in modern times that Hallie and Hayley have become confused. [noted -ed]