On the writing credits, my edition of Gershwin songs lists only duBose Heyward as the writer (with George as composer), although on the ASCAP copyright data base Ira Gershwin is listed also. However, Caryl Brahms & Ned Sherrin in their book Song By Song note that it was George who put forward Ira to collaborate with Heyward and insisted on a 50-50 split between the two lyricists. They also note that "In some of the songs he simply admired what Heyward had written ("Summertime", "My Man's Gone Now", "A Woman is a Sometime Thing", "The Buzzard Song")" and go on to list the songs in which Ira "edited the words by making sure they were singable", "enjoyed a full-scale collaboration with Heyward" and in others Ira was the sole lyricist("There's a Boat Dat's Leavin' Soon for New York", "It Ain't Neccessarily So")

On the lyrics, the second version above agrees with mine in most respects, except, mainly, that lots of final letters are lost, so it looks like this below.

Mick

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SUMMERTIME
(Music by George Gershwin, Lyrics by duBose Heyward)

Summertime an' the livin' is easy,
Fish are jumpin', an' the cotton is high.
Oh, yo' daddy's rich, An yo' ma is good lookin,'
So hush, little baby, don' you cry.

One of these mornin's you goin' to rise up singin',
Then you'll spread yo' wings an' you'll take the sky.
But till that mornin' there's a nothin' can harm you
With Daddy an' Mammy standin' by.