There is an official
Guided Snorkel Program on Bonaire. They picked 5 sites to highlight:
The Mangroves off Lac Bay - not really what you want in terms of seeing rays/turtles or bigger fish - also a drive to the S.E. side. Might limit your in-water time.
1000 Steps - a little north though.
Cliff
No Name at Klein
Just a Nice Dive at Klein
1000 Steps has those 64 stone steps to walk down - and back up...might actually be a better snorkel than dive for that reason - less gear to pack...LOL.
Scuba Shore Diving Site Page for: 1000 Steps of Bonaire North, ABC Islands It's an easy beach entry after that...you will want to heed the vehicle theft warnings there since you're leaving the truck up the hill and across the street. It's a little far north also with limited time.
Cliff is accessed off an area just north of Habitat - the next road you come to that goes to the left. There's a little public parking area. Be mindful of the private properties in that area. Often there will be other vehicles there, Cliff is also a popular dive site. From there IDK how deep the snorkeling is, we swim up from Habitat so are already in deeper water when we hit the wall.
Scuba Shore Diving Site Page for: Cliff of Bonaire North, ABC Islands
A 2nd snorkel stop nearby is south Bari Reef. Just south of Den Laman Condos is the public access point for Bari Reef - the turn-off just past their entrance. Bari is the #1 site for counted fish species in the Caribbean. There was a stone pier there once and a resort - both "relocated" by a hurricane. I snorkeled around in the shallows there after a dive - only about 5-10' deep and a lot of rubble - and remains of the pier still underwater. Lots of fish but not a lot of bigger stuff. We saw squid there once near the pier.
Bari will often be calm when other sites aren't. Bari's dock is private property and leased by Bonaire Dive/Adventure. There's an on-going legal battle between the owner and the dock owner so I've read they tightly restrict access only to guests at Den Laman excluding Sand Dollar Condos usage. But anybody can park in the field and walk in.
Snorkeling Bari Reef - See Fish At A Typical Bonaire Town Spot
With only 1/2 day although you can get out to Klein it might be tight. Maybe try one of the cruise snorkel operators? Renee gets good reviews:
http://www.infobonaire.com/reneesnorkeltrips/index.html
My other choice would be Windsock/Donkey Beach just off the airport for novice snorkelers. It's a very easy, flat entry and you may see some turtles/rays there. Google Bonaire snorkeling - it's on a lot of lists. A few minutes south of Town Pier where your friend's ship will dock.
Scuba Shore Diving Site Page for: Windsock of Bonaire North, ABC Islands
For something else touristy nearby, the road to the Donkey Preserve is near there also. If some of them got out, (happens often) they may be hanging around nearby - bring apples for your new friends...you can also visit the Preserve.
Your friends should at least bring surf walkers with rubber soles if not (at least) lightly treaded boots/open heel fins. People snorkel in full foot fins which would be difficult getting offshore at a couple of these locations.
hth,