I've been living here for the better part of a year and diving as much as I can.
Don't know about Lembeh in January. Just be aware that it is the muckiest of muck sites. There's little to see besides black sand and tiny critters. You need a LARGE camera or at least a magnifying glass. If that's your thing -- you'll love it. If it's not, you might decide to leave early like we did.
Bunaken had great walls with corals and lots of turtles. Bangka is also nearby, great coral reefs, there are several resorts.
There's a
lot of trash in the water in both areas.
Dec 28 - Jan 9 is less than two weeks. I tell my friends not to come to Indo unless you have three weeks. 24 hrs to travel, 24 hours to recover...the same on the other end...now you're down to 10 days at most. Just think about whether that is worth the money, etc to you.
Contrariwise --- Indo has the best diving in the world. You might hate the Caribbean after diving Indo!
Have you considered Raja Ampat? A bit harder to get to, more expensive, but SUPERB diving.
And many places are already booked-up for this year.
Bali has very good diving, though not as good as Raja Ampat or N. Sulawesi IMHO. Can't see why the rainy season should be a concern at all, at least not for Amed & Tulamben (those are dry regions of Bali). You can also station yourself in Padang Bai and take boat trips to see mantas and great coral.
Hope this helps. Now here comes the chorus of posts telling me how great Lembeh is.
- Bill