Practically in your situation, if you're not interested in full frame housings, is to get a better lens/port combo. Even upgrading to the A7RV sensor, not many lens/port combos can resolve to the complete 61MP, especially at the corners.
A7C2, 28-60mm w/ WWL-1B, 1/80s, f/8, ISO 100, no strobes, centre crop, corner drop and side crop
A7C2 (33 megapixels, 7008 x 4672), 28-60mm, WWL-1B, no strobes
1/160s, f/9, ISO 500
100% crop below. It probably is sharper than the RX100, but we're dealing with a bigger sensor, much more expensive wet lens.
Just a quick google search:
https://www.backscatter.com/reviews/post/Sony-a6500-Camera-Underwater-Test-and-Review
They suggest the Tokina 10-17mm with an adapter.
Don't these two sentences contradict each other then? I think everyone in the comments are replying to your initial assumption that your photos are not sharp because of the sensor, which I think my example can help disprove.
I agree that it may be a combination of your lens and dome combination, although just looking at the Nauticam port chart, it already is the "ideal" solution. Of course, there may be sharper options out there for wide angle, but unfortunately, I'm not too familiar with the APS-C options out there, so others may be more helpful.
Planning a trip to Ishigaki Japan in April, but the budget airline I'm taking is notorious for their 7kg carry on limit. I'm looking to save weight by using a duffel instead, but was wondering what other options people are using especially regarding weight.
I can confirm the Backscatter nauticam trigger works well with my A7C2 with nauticam housing. The TTL also seems to work well on land with the HF1s. I do like the fact the nauticam trigger is designed without wires, making it feel more durable, although it may be less viable for alternative housings.