Some searches lead people to places that no longer exist, and a guide that quietly omits them is not being helpful.
Tullis-Toledano Manor was an 1850s brick house on Beach Boulevard and it was destroyed in 2005 when a moored casino barge was driven ashore into it during the surge. The site is open ground. It still appears on older lists of Biloxi attractions.
The old Biloxi Bay Bridge was wrecked in the same storm. Its replacement opened on 1 November 2007 with the walking and cycling path that is now one of the best free things in the city, so this is one of the cases where the rebuilt version is better than what was lost.
The barge casinos are gone as a category. Nothing floats any more. If you have a memory of walking down a gangway onto a gaming floor, the law changed after 2005 and the floors are on land now.
There is no snorkelling or diving here, and no operator will sell you any. The Sound is shallow, turbid and has nothing to look at underwater. Anyone promising clear-water snorkelling on the Mississippi mainland is describing somewhere else.
There is no cruise terminal, so there are no shore excursions, and the tour economy is correspondingly small: the whole destination supports roughly eight bookable products.
Keesler Air Force Base is not open to casual visitors. The Hurricane Hunters fly from it, which is genuinely remarkable, and you will not be seeing them.