Hi Benawa, tipping is not the norm here on Rarotonga. I'm not sure where you normally dive, but NZ$50/boat dive is not expensive when you compare to other places in the region. We do have everying imported, including fuel and power is extremely expensive. The other thing is that in the Cook Islands staff are paid a semi-realistic minimum wage - so you can't compere to places like Asia where staff are paid extremely poorly and live on tips.
As for the diving, Rarotonga is a desination with diving - not a dive destination. That said we have exceptionally clear water (30+m pretty normal) and we do certainly have plenty of coral and fish.. so I'm not sure where people are diving??? I suppose it all comes down to what you expect. If you expect the Red Sea you'll be dissapointed, but compared to many placed in the South Pacific, Rarotonga has better dives. The best coral is at sites like Mushroom Forest and Swiss reef (North), the West is the place for carverns/swim thoughs and the South is good for passage diving athough here the coral is not so good - perhaps this is where clownfishsydney was diving? BTW The coral on the island is virtually all hard coral.