suggestions for the next lens purchase with UW as the goal?
This is a fun topic, I love spending other people's money!
The above recomendations are great, here's the lenses I recomend in the order I would suggest buying them.
IMHO, for the first 2 U/W lenses, most try to cover wide angle and macro ranges. In "good glass" terms, you can cover these pretty cheaply with:
Tokina 10-17mm (the Nikon 16mm and 10.5 are both redundant with this amazing lens)
Nikon 60mm
These are great lenses, you can get both for well under $1000
If you want a mid-range zoom, you cheap option is a Nikon 18-70mm which can yeild decent results with the proper dome/ext ring/diopter combo. I use it often with nice results.
For the more expensive end, as mentioned above the 17-55mm is a very sharp, good lens, much better than the 18-70mm, but 3 to 4 times more expensive.
For a rectilinear wide zoom that will get plenty of use topside, the 12-24mm is a great lens. The Nikon is slightly better than the Tokina, but almost twice as expensive. I love the Tokina which I own.
For more specialized macro, the Nikon 105VR is a great lens, but almost everyone starts with the much easier to use 60mm before adding the 105mm to their kit. If you eventually do buy the 105 you will still find plenty of use for the 60mm, so don't worry about buying a lens that you will outgrow.
If you happen to go with Sea & Sea, I can tell you how to set up each of these lenses for good results.
Happy shopping,
John