Some places to try - I dive but these seem like good shallow snorkel options also.
Bari Reef - I have snorkeled here - actually floated around on Scuba. There's a lot of hurricane rubble just off to the left of the dock - maybe 3-5' of water. Saw a lot of fish there - literally dozens of species in that area. Squid also twice.
Start this panorama -
http://www.bonairepanoramas.com/panorama/bari-reef.html - the area I'm talking about is in the foreground. The pink/green building is Den Laman condos for reference. Spin around to the left to see Klein - my guess is that's about one of the closer points to access it from. Check with Bonaire Dive/Adventure while you're there - they do kayak trips to the Mangroves.
Windsock Beach by the airport is probably good. In some areas the coral almost breaks the surface. Easy entry.
Salt Pier - the 1st couple of pilings are less than 20' deep. Some have fire coral on them though. Check the panorama here:
http://www.bonairepanoramas.com/underwater.html
It's one of the mid-level piers - maybe 25' deep? - and there's one closer. Even the farthest pier is only about 45' deep IIRC.
Karpata to the far north of town - there's elkhorn coral just right of the old pier. Once you commit to going there the road will become one way so you'll have to drive back thru Rincon to town - about 1/2hr. drive. But interesting. Lake Gotomeer is also in that area if you want to see the Flamingos.
BelMar's not bad either - right under both docks but the surge can push you into the ironshore. IDK if they still do but at one time they had lights under the dive pier which attracted lots of small fish at night - and the occasional tarpon feeding on them. There's always lots of little fish in the shadows during the day - once about a dozen squid.
IIRC there was a patch of larger coral heads at about the 10 o'clock position if you're standing on the south dock facing west. Pretty shallow like 15-20'. If you get out more than 200' you've missed them. There's also some big purple tube sponges but they might be a little too deep. Ask the Buddy dive folks on-site.
I've heard they snorkel in Lac Bay also. But it's about a 20min.
walk out to deeper water - dodging windsurfers - so IDK. They kayak in the Mangroves nearby also.
If you do get over to Klein, South Bay would be good snorkeling. It's both protected and pretty shallow. We saw turtles there inland from the mooring under the rocks - it's about 20' deep at the mooring ball IIRC. I swam towards shore once and it was almost was too shallow - I was afraid I'd hit the coral. There's occasionally dolphins in the channel between the two islands - I've talked to divers who snorkeled with them. Seems like late morning is when they saw them. We also saw them swing by BelMar late one afternoon but they were pretty far out.
A few other options:
Info Bonaire - Other Great Snorkeling sites around Bonaire and Klein Bonaire
hth,