On a photocopy of the Maughold parish register dated 31st December 1868 I have the following marriage, entry 460 :
John Christian to Elizabeth Garrett, brides father: William Garrett, occ: Pilot
witnessed by Robert Garrett.

I hope this is of use to you.

I am not researching the Garretts but in 1863 my g-grandfather John Curphey was the miller at Garrett's Mill, Sulby. Are your Garretts connected to this Mill?