I was thinking a tamron 17-50mm f2.8 as he doesnt want to spend a fortune :shrug:
Pretty much useless for underwater and unlikely to fit in many housings.
Firstly is it underwater macro or wide angle? The 2 are completely different fields needing different ports and so on.
Underwater distance to subject is everything as the water absorbs light and colour massively. There is no red light below about 5m for example. Even the most powerful strobe is going to fail to do anything with a subject distance over about 1.5m.
So far that reason generally you need a lens as wide as possible. I use a Canon 10-22mm f/3.5-4.5) and its very common. Nearly all my photos are shot at 10mm as you really do have to be that close to get decent colour even with a big strobe.
Some people use the sigma or other lenses but certainly 10-20mm range is where you need to be. Aperture isn't really an issue due to the air-water properties and subject distance.
A lot depends on the housing as well - some lenses (such as sigma) wont fit on the zoom gear in an ikelite housing for example. Some need dioptres to improve corner sharpness.
A 17-50 lens will put the subject too far from the camera to get any sort of good light and colour and its too "wide" for a macro setup. Non-starter really. Also the new USM version is too thick in size to fit into any housing i know of. You need to see what lenses fit into what housings rather than buy a lens and find it wont actually fit into any housing.
A 10-22 or similar will work perfectly for landscapes as well (i use it for both).
As well as that he'll most likely need a strobe - good ones come in at £800 or so. Eventually 2 or 3.
Underwater photography isn't cheap. Quite often the body and glass are the cheapest part of the setup. A cheap housing alone will cost about £1500, then add strobes and so on don't expect change out of £3000.
My setup is 10-22mm for reef/coral/diver shots and Canon 60mm f/2.8 for macro shots (ie really small shrimp and so on).
Macro is pretty much impossible without a powerful strobe whereas wide angle IF you keep it shallow (less than 10m or so) and in tropical clear waters you can get away with it for a while before getting frustrated.
I got the lens first, used it on land for a while before deciding which housing setup to go for and spending the money. Need to choose from Ikelite (cheapest but bulky) through to the more compact and pricey Nauticams, Subals etc.
Edit:- Dont think this is against board policy, i have no connection with the company but a link to get ideas about housings, equipment needed etc:
http://www.camerasunderwater.co.uk/slrindex