Riker and David from Sydney,
I have never felt the need for FF and shot for years with APS-C Canon and many fisheye lenses which all can be sharper in corners even with a smaller 6" / 140mm dome than most rectilinear lenses. Any middle curvature can be "de-fished" or flattened in software easily.
From 2016 - 2025 I tired of hauling housings, strobes and shot exclusively with compact cameras. Mainly the Canon G7X II plus various iPhones but always wanted to get back to APS-C.
I currently own two Canon R100
(the most despised made Canon in recent years) but produce great files with a decent lens. It's super cheap for traveling and shooting underwater. I justified a 2nd one purchased on the Canon refurbished site. with the "kit" RF-S 18-45mm IS STM lens and RF-S 55-210 IS STM for a ridiculous $429.00 USD. Not being a long lens shooter I sold the EF)S 55-210mm IS STM and spare 18-45mm netting a spare body about $180.00 (!!!!)
I also own a Canon R50 and the AF CMOS II, tracking and other features would have made it my first choice to house but Ikelite didn't tackle the 21 pin hot shoe connector
I use it as my main surface camera with the fabulous RF)S 18-150mm IS STM lens.
The lowly Canon R100 has a Canon "standard" 5 pin TTL hot shoe as does the R10 / R7. My Ikelite Canon R100 housing has a TTL fiber optic cord transmitter which I used for an article and their new small Ecko Fiber strobes:
www.divephotoguide.com
I did own a Nauticam NA-R50 for the Canon R50 for about 5 months but sold it... I didn't like being locked into just using "wet" expensive lenses you bayonet on outside the fixed housing's port.
The Ikelite DLM housing for the Canon R100 allows me to use multiple lenses. I'm strictly a wide to medium fish shooter these days, no more macro after 55 years diving this year......
The housing's interchangeable 6" dome port accommodates the underrated Canon RF-S 10-18mm IS STM and RF-S 18-45mm IS STM lenses. It will also allow the discontinued Tokina APS-C 10-17mm Fisheye with Canon EF-RF adapter. I have access to those from friends and may take it to Palau end of next month as a back up.
I could also use the discontinued EF 8-15mm L F4 with EF-RF adapter and a slightly longer dome port. But installing that large a lens requires inserting camera body from the back then lens, zoom gear and all through the front before sealing the port.
You have to do that EVERY TIME for changing batteries or memory cards. For me that's a deal breaker no matter how much sharper either the Canon EF 8-15mm F4L or new R F7-14mmL might be. I won't be justifying that cost, especially on an APS-C sensor.
My advice to Riker or others if you really require FF is just get a 6DII / III and new 7-14mm is your budget allows depending on your end photo use is.
On APS-C sensor Canon cameras I usually bought rectilinear lenses as I'd use them above water. Only the less expensive Tokina 10-17mm was used for a few years but I also tired of the curvature look. Many love it and some don't....
The Canon RF-S 10-18mm IS STM is actually damn near perfect behind the 6" dome on my Ikelite DLM housing for edge too edge sharpness. Partly because of APS-C sensor depth of field over FF and 10mm on APS-C is around 16mm which is about the widest one can get sharp corners behind any dome. Unless you're one of those who try and shoot f11-13-22 all the time which are flash shooters mostly. I can shoot even f-8 in bright conditions using ambient light tropical shooting and even at higher ISOs the files look fine.
Sorry for the long write up, just sharing insights from decades of looking for a good combination of performance to cost
David Haas
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) and even better pics 
