Gwangi, I've always been a user of high-quality primes because of my prior job. So I'm prejudiced, take what I say with a grain of salt.
The Nikon 14mm is quite a bit better to my eye than the Sigma or Tamaron, (certainly more tack wide open in air) but whether the Ikelite optics will give you the return is questionable. Aquatica, Subal, Seacam, etc... you'd notice it if you're critical, as a recreational shooter maybe not.
Anyway, I was struck by a thought, and tried to slip my Nikon 14mm into an (admittedly old, early 90's vintage) Ikelite dome port, and it would not fit through the throat. It's just too big. Ike may be making this bigger, though, my Ike is kind of old.
If the size is still the same, your decision is narrowed for you.
All the best, James