This past year I was worked in the St. Vincent Grenadines as a diving instructor out of Union Island, we frequented the famous Tobago Cays
and other small nearby islands such as, Petit St. Vincent, Petit Martinique, Mayreau, etc... 
The diving as far as the Caribbean goes was excellent, some of the reefs especially the dive site called Mayreau Gardens is truly pristine,
others though have suffered some damage by a major hurricane a few years ago, however don't let that stop you from going.
The diving is amazing, you can see grey reef sharks in their natural element without anybody feeding them, which is sadly a rarity now a days.
Lots of nurse sharks, big lobsters, nudibranchs, WW1 wreck (called the Puruni is an amazing night dive only at 40ft), loads of turtles, eagle rays, and everything else you would expect to see.
The must dos are Mayreau Gardens and The Puruni (at night).  Carriacou is an island just to the south of Union Island
and is largley untouched and also has some amazing pristine diving, its part of the Grenada Grendaines.

I also had the chance to visit St. Vincent (the mainland) which is a very special place, if you like macro this is your Caribbean destination, I saw free swimming sea horses (will post video), pregnant sea horses, juvenile frog fish, peas, full grown frogfish, multiple different species of pipefish, many different nudibranchs, mantis shrimp, and the list goes on, whats suprising though is this was only in ten feet of water at the dock that the boat we were staying on was moored up to, we didn't even do a dive at a proper dive site.  I really want to go back and dive the island properly, only did about five dives in very shallow water, but they were amazing as far as what we saw. HIGHLY RECOMMENDED

The ideal way to do the Grenadines is to have unlimited amounts of time, however that's not possible for most of us, so what you should do is fly into St. Vincent (a bit of a treck as there are no direct flights from anywhere except other islands, Barbados has daily connections, thats one of the reasons the diving is so good) and stay for about a week and do as much diving as you can including night dives every night (they are amazing).  Then catch the ferry to Bequia to do some more diving (Bequia is also known for its macro life), I never made it but it is a must do, then you should head to Union Island and dive with Grenadines Dive, they are optimally located to go to each of the surrounding islands, the Tobago Cays are a short 15 minute boat ride away.

Do not go on a cruise ship as they anchor on the reefs and dump there waste onto the reef, and you wont get much diving in.  These islands are a sailors mecca, if your on a sailboat please don't dump your waste near the reef, not even banana peels as they contribute to the algeafication of the reef.

By Fontaine