I agree with Alex .. being an islander (currently away from home but back in a week or so) myself It is very rare to use anything other than wide or macro .. just depends on how far you want to go either way.
I have never had a lense choice before now and I have opted to stay away from a middle length zoom for now (i have one just working on how to house it, anyone had the canon 28-70 EF 1:2:8 L usm (77mm diameter) in a sea and sea housing?)
The only time I can remember using the flat port on my 3040 (which is equivelent) was when I was starting out and when I was trying some dusk dive shots with Alex as I couldn't get close enough with the other lenses.
Anyway .. basically I agree with Alex .. if you are sticking to Cayman the 10.5 and the 105 will be great lots o people use the 60mm in cayman too but I always think that mostcayman macro shots require closer, but as you have the 60mm I would consider getting a port for the 10.5 and that lense next you already have a macro so get a WA, think of the eagle rays on the north wall in the summer time, the sharks chasing the eagle rays, the beautiful wall shots all over, the tarpon at turtle reef, the classic eden rock shot, stingray city moments, the oro verde, babylon in all it's glory !!!!!! can you tell i miss it yet ... almost home my loveable rock i'll be back by the end of next week i promise !!
(btw .. with my previous 3040 setup i never got wider than 24mm which did me fine for the most part, but after a short while of using it I wanted wider, samew with the macro, i think the macro port I had gave about 1:2 which is about 60mm - 70mm i think but I always wanted closer always, thats why for my new setup i got the 15mm fisheye and the 100mm macro straight away)
ps who do you dive with in cayman ... i'm wondering if we have ever bumped into each other ?