Liz, are you using the 14 I or II? I tried the 14 I and compared it to the Sigma version, and the Sigma version was sharper and had slightly better corners. However, neither had good corners before f7 with a 230mm dome. What kind of corners are you getting with the 8" dome?
I have tried the II but it doesn't impress me as much as the Samyang, but the Samyang is manual EVERYTHING, which makes it difficult to use in quick shooting circumstances.
I'm using the 14mm II. The reviews for the I weren't particularly good, although I haven't tried it. The autofocus for the 14mm II is relatively good in the (very) low light underground. I do find that the corners on the II can be soft before f8, but I think this is caused more by the dome than by the lens itself The shot below was taken at f6.3 and not cropped...although my determination last year to shoot everything at a low ISO meant I had to lighten it up a bit in Lightroom. My butt is almost against the opposite wall to get this angle, and the diver in blue is about 1.5m away from the dome from memory....20mm wouldn't have worked for me.

Bathtub by Liz_Rogers, on Flickr
On the other hand, I often/usually crop the very edges off my shots. And in the ocean there may be more opportunities for backing up to frame your picture.
All of this can be taken with a grain of salt - I haven't experimented with much else except what I'm currently using, and I love my rig. Good luck with picking something that works for you!