OK here goes. Some comments. Lenses are very personal and choice depends very much on what you enjoy taking photos of.
I own a 15 Canon but don't like fisheyes myself so rarely use it - remember that lighting can be a big factor if you go for a fisheye so your strobe(s) choice is important. The Sigma has a good reputation. Build quality of the Canon is probably better as is its resale value (goes for all Sigmas/Canons).
I've used Canon's 20/2.8 and 24/1.4 primes underwater. The 24 is by far the better lens but the 20 works surprisingly well underwater (ie its soft corners don't get much softer!). The 28/1.8 sounds interesting and I'd like to try this one myself - fast lenses give brighter viewfinder images. For these lenses you don't have to use a huge dome provided a smaller one is matched up well - most manufacturer's will have data on recommendations about their smaller domes and prime wide lenses.
I've also used the 17~40/4 which works well enough although its not up to the 24/1.4 - versatile though - check a zoom gear is available from the housing manufacturer you go with and a bigger dome is better. The 16~35/2.8 is faster and more expensive and by all accounts no better optically - I've a friend who uses one commercially and has no adverse comments about it.
The 24~70/2.8 isn't bad but would need a diopter to bring focus to anything like reasonable I would expect.
If you go for a Sigma Zoom - again check zoom gear availablity.
Macro - good choice of:
Sigma 50 - goes to 1:1 but I've always found Sigma 50s to be noisy and have had reliability issue (personally that is) - some people really like them and their optics are pretty good.
Canon 50mm - only goes to 1:2 but works well underwater - its useful in not so good conditions. I quite like it within its limitations. Very cheap for a lens of its optical quality.
Canon 100mm latest - stunningly good lens - by far my favourite macro lens to date. Both Canon's 50 and 100 macros have 'L' performance.
Caon 60/2.8 - with a 12mm extension tube this lens give a restricted focus but promises a lot - I've just got hold of one and will try it as soon as the mud which passes for sea near me settles down a bit.
Haven't bothered with any other macros - savings are not huge over the Canons and given the quality of the 100/2.8 I'd be hard pushed to recommend anything in its stead.
Lastly, I was talking to someone who is using the new 24~105 IS underwater (custom built gear/large dome) and he finds it very good - especially likes the IS at the wider end. If I can get a gear......
