Hi John,
Hope all is well in Cayman. It sucks being stuck in the UK this summer.
The 17-55mm will need an extention ring (behind the domeport) and a dioptre to work best underwater. I don't believe its an ideal lens for Cayman UW - although its a great topside lens. 10.5mm and 105mm are my choice weapons in your seas.
My camera has the same sensor size as yours and I use UW the following lenses:
10.5mm, 16mm, 12-24mm, 17-35mm, 28-70mm, 60mm, 105mm.
I realise we all shoot in different ways, but in Cayman I would say their order of importance is 10.5mm, 105mm, then a gap, then 28-70mm (for similar stuff to your 17-55mm), then 60mm, then a gap and then 16mm, 17-35mm and 12-24mm all about the same.
Hope this helps.
Alex