Thornback cowfish from this afternoon in Komodo.

I just bought a salted line seafrog housing for my a6700 camera with a 4" medium flat port (90mm) to fit my lens.

but i also have a macro wet lens that i bought from before, with a 67mm thread fin. How can I use it on my new housing? I searched around and it seems like most adapters don't do 90mm - 67mm.

HELP!!!

Looking for some advice on getting into underwater photography.

I mainly shoot marine/yachting photography at the moment. My usual setup is a Canon R6 Mark II, with an R7 as my backup, and I mostly use the 100-400mm and 70-200mm.

I’d like to start experimenting with underwater photography and have found a relatively cheap €1200 second-hand setup consisting of a Canon RP, SeaFrogs housing, dome port and the other bits needed to get started. From what I understand, I should also be able to pair it with my Canon 16-35mm f/2.8, which seems like it could be a decent underwater setup.

I’m considering going this route rather than buying a housing for my R6 II, mainly because it would let me get into underwater photography relatively cheaply without putting my main camera at risk while I’m learning.
For anyone with underwater photography experience: does the RP + SeaFrogs setup sound like a sensible starting point? Are there any major drawbacks or things I should look out for when buying a used housing/dome setup?

Also, would the 16-35mm f/2.8 be a good lens to start with underwater, particularly for boats/yachts, marine life and wider environmental shots?

Any advice for someone just getting into underwater shooting would be greatly appreciated!

I've been doing some underwater snorkel stuff for a handful of vacations now with my D850/ikelite and 14mm-24mm lens to keep things "more affordable" to see if I even like it. I don't dive, so it's just surface stuff. Learning curve is big, but it's been enjoyable, maybe cumbersome is the most annoying aspect.

I'm returning to a place with turtles as the main attraction (Akumal) and thinking about some upgrades. I can walk right off the resort beach straight into turtles, so lots of turtle time to make a new purchase worth it.

One of my biggest gripes with the underwater stuff is the cost to even get out onto the water and how rushed the normal snorkel times are.

What is the next step up for a lens? Fish eye? What about a straight port option? Everywhere I've been so far, just not close to a lot at the surface.