I've just checked out 'Blackwater Park' and 'Ghost Reveries', so I'm just getting into them. Still getting used to the "cookie monster" vocals in places. I've been in a very heavy and aggressive mood music-wise lately.
I totally get you. Opeth was my gateway drug in the sense that I had never really listened to any music with growling vocals like they had when I first got into them about 10+ years ago. At first, the vocals were a real stumbling block for me too and I just couldn't get used to them. But I kept getting drawn back into listening to the band because I found that no band I'd listen to could achieve the contrasting dark/light atmosphere within the same song the way that Opeth could. I couldn't believe that it was the same guy singing both clean and the death metal vocals, loved their use of acoustic guitars to layer in, etc. So one day even the vocals just clicked for me and now I can't imagine Opeth without them, although they were an acquired taste for me too. I do respect Opeth's evolution into an all out prog band and, as I mentioned, their latest release is nothing short of brilliant, but I do with they'd go back to their old sound unexpectedly, at least for one more album.
You're in an enviable position as a new fan. You have so much more to look forward to. You definitely still have to check out Still Life and My Arms, Your Hearse. Those two, along with Blackwater Park are my top three. But there's still Morningrise, Deliverance and Damnation. Listen to the latter if you're in the mood for the lighter side of Opeth especially. There are some stunning songs on there for sure. Hope you'll continue your musical journey with Opeth and let me know how you get on - as you can probably tell, they're one of my absolute favorite bands.