Manado and North Sulawesi should be OK for Westerners. There are now conflicts in Ambon in Maluku, and things in Poso, Central Sulawesi can be uncertain. But Manado is a modern enough city, and should be safe.
The general advice is be polite, don't attract too much attention to yourself, and keep smiling during encounters with locals, including the authorities.
If it's worth anything, I'm an American journalist who has lived in Jakarta and covering the news here for the last four years.
On the diving issue, I'd suggest Froggies Divers (
www.divefroggies.com) on Bunaken Island itself. It's run by Christine, a Swiss (if I'm not mistaken) who has tons of experience in the area and is one of the people who are active on the conservation programme there.
Staying on the island means easy 10-minute boat rides to your dive sites. You can go back to the island and relax during surface intervals, and then decide at your leisure when you'll go diving again.
Staying in Manado and other points on the mainland means hopping on a boat and travelling 1 hour to dive sites, diving, then sit around in the boat during the surface interval, then diving, and then another hour on the boat back to your Manado hotel.
I don't know about you, but I'd rather be dry and sipping a cup of tea with milk under a tree with a book on my lap while waiting for the next dive, and not inside a cramped boat, still in my wetsuit, cold and wet.
