Nancy, I don't yet have a definitive answer for you but this information may aid your search.
I don't find him with any title, other than Gent, through his marriage date 1663. Side note: I see that the date of marriage appears to be 1660 on the wife's profile but most records I am seeing are showing 1663. You might want to source and correct that for both profiles.
Later in life I do not find him given as a Capt Booth with one exception so far found. In a transcription of Virginia Land Book, Richmond, Book VI, 539, "September 1672. Nicholas Catlett and William Moseley, 1,200 acres, adjoining Capt Humphrey Booth, south side of Rappahannock." This is a transcription so my suggestion now is to see if you can find a different transcription (Library of Virginia online is the place to start) and see if it also reads Capt. If it does not, then I think it is likely an error. If it does read Capt then he became Capt of something between marriage in 1663 and this land record in 1672 which narrows the search window considerably. I see no reference to him as a mariner which leaves it to be a military rank if he was a Capt.
Also, go back to your question and click the Edit button. Then enter both Virginia and southern-colonies in the tags, then save it. That will bring this question before the Virginia team and the new Southern Colonies team focusing on Virginia.
