Get the 60mm macro. It makes a great pair for the 15mm FE which is also a good choice for a good, cheap, simple WA.
Your two choices are the 60mm or the 105mm. The 60mm is better for most fish portraits. Its my favorite lens. I get most of my keepers with it. It focuses fast in low light, is less expensive and is perfect for subjects from butterfly fish sized to turtle size. They both get 1:1 so you can even get closeups of static subjects like Christmas Tree Worms just as well with this one.
The 105mm is more of a specialist lens. It gives you more breathing room to light the subjects and not spook it, but it doesn't focus as well (even the AFS VR version), the FOV is often too narrow for a lot of subjects, and its harder to steady the long lens. The 105mm (or Tameron 90mm) definately has its place but its not the first macro I'd get.
Many of the Sigma zooms are listed as "macro" lenses. Don't believe it. They have a relatively long minimum focus distance and can't achieve anything close to 1:1. They also lack the sharpness or fast aperture of a true "macro" lens. This is all about marketing.