8" dome is great for fisheye on FX. Main benefits of bigger dome are for splits and rectilinear lens.
I would suggest buying the 16-35mm over the 14-24mm for underwater use. Reverse is true on land. But 16-35mm is better behind a dome IMO.
This image taken on FX in very dark conditions with available light only with the 16-35mm (using Zen 230 dome) and I think the corner performance is excellent.
I just ordered the 9in optical glass dome from Nauticam for their D800 housing (April I'm told) with a view to using firstly my 14 -24mm then (once I recover from the cost of the dome!) the 16 -35mm as you recommend. What are your thoughts on this domes use with these lenses.