Dive specific sites like ScubaBoard or Undercurrent are a good place to start. One thing about a site like ScubaBoard is there can be a high degree of group-think, where people on it tend to go to places others have recommended already, so some good places can go under the radar. Undercurrent seems to make an effort to check out less common places, but they can only hit so many.
Google, of course. You will find other things this way, but some of it will be junk like any search, with more research required. (I see stuff pop up in search for place that I know closed long ago or is just completely wrong. Stuff lives forever.) A good looking website for an op is promising but no guarantee of anything. A responsive op is also a good sign but also no guarantee. Non-response, while problematic, is not always a reason to skip a place.
Sometimes tourist sites for a country have good listings that can help, but don't always turn up in a search unless you are looking more specifically for them. (I imagine many of the people doing these sites are not well funded, they may not have anyone that knows the tricks to get their sites up higher in search results, never mind anyone staying on top of things and updating them.)
Sometimes dive travel agencies have some good listings to get ideas, I'm probably more inclined to look for that sort of thing in areas I'm unfamiliar with and not coming up with much otherwise. There are also dive travel sites out there that seem to be ghosts, again with listings for places I know are long gone.
I don't bother with general audience sites like TripAdvisor for dive travel. The people that talk about diving on there tend to be looking for something different than I am, or have a very different idea of what is good or bad. Now for an add on to a dive trip, definitely.
I have lots of bookmarks or am on mailing lists for dive shops or others that organize group trips we might be interested in that I check on once in a while.
Over time if you read and look around enough you should develop a pretty good general idea in your head of wha'ts out there anyplace you might consider going. Really that's my main starting point now. One thing is for sure, I have no problem finding candidates for places to go and have a long list...