The working distance of the 70-180 varies but it gets under 5" in the middle of its focal length. At the 180mm end, its effective focal length shortens to around 100mm at close focus. It is capable of 3:4 at 180mm. With diopters it responds well and can get to 1:1 or even better.
With the 70-180 you can get nearly all the fish portraits of a 60, most of the reach of a 200 for shy subjects, and much of the magnification at close distances of a 105. Not as good as any of those lenses for their intended purpose but you get all of them on the same dive. Image quality is great, too, but autofocus sucks.
Because of the focus range limitations of the Tamron, the trick will be for James to figure out how to focus close enough while still maintaining enough flexibility to reach those shy creatures and shoot the larger subjects. Without being able to shoot fish portraits, there's not much to argue for the zoom over simply using a 100 or 150. I believe he will need to fix a +1 and use a macromate or Inon wet diopter. Trouble with that is that I'd hate to have to combine 200mm, a wet diopter, and autofocus. 