My wife and I have been to Cozumel more than a few times, alone and with friends. Also, St. Lucia, Curacao, Turks and Caicos, Hawaii, and Bahamas (cruise with family). There may be one or two other places. I've also been to Coz a few times by myself.
I do not recommend Bonaire unless your SO is willing to sit and watch your stuff on the beach while reading a book. That's because it's mainly shore diving (fantastic) with little other activities to do.
I am not solo certified, nor do I want to be. All the places I've mentioned will keep you in a group (with the exception of Bonaire. I went with a dive buddy). I will say I have surfaced more than a few times by myself in Coz because the DMs knew I could handle it.