You actually have several compelling choices of 24mm lenses for sony FE full frame. The new Sony FE 24mm F/2.8 has excellent weather sealing, an aperture ring on the barrel and a programable button. These are all useful above water but useless in a housing. The lens is also $598.00 new. I would also suggest Looking at the affordable Rokinon 24mm F/2.8 and the Tamron 24mm F/2.8. Tamron is now on sale for $199.00 and the Rokinon on sale runs around $250.00. I reviewed the Rokinon 24mm for UWPMAG.com issue #116 and I suggest you take a look at that review for some insight into how useful the 24mm is underwater.
Sony and Rokinon focus to 24cm, Tamron focuses to 12cm which is 1:2. The Rokinon is quite good and well worth the price. All three lenses should work behind the ZEN 170mm or Nauticam 180mm ports.
You may also want to checkout my review of the Rokinon 18mm F/2.8 which will also work with the 170/180mm ports, it is in the current issue #119 at uwpmag.com, the magazine is a free PDF download with all back issues.
Bottom line is I would own the more expensive Sony FE 24mm if my main goal was above water use with occasional U/W use, otherwise I would be choosing the less expensive options. Of the four lenses the 18mm is the most useful underwater