I understand that this isn't the ideal lens to go buy for UW photography, or the most popular choice for focal length (and fixed on top of it).
I really don't know why. I find fixed focal length lenses make you really think a lot more about how you shoot. I've used 20s, 24s and 35s underwater and I'm thinking about using a 28 in the near future. Many are excellent lenses and can be set up to be very good underwater. Limiting, perhaps but actually are (as above water) a very good discipline, and can give a better appreciation of how to use a specific focal length (and FWIW I've not shot on a zoom for about 5 years below or above water and I do think that this has improved my photography).