Lensrentals offer some underwater housings via
https://www.lensrentals.com/catalog_search?q=ikelite for rent but otherwise you need to buy it outright. Minimum you need for a DLSR/MILC is the body housing, the port extensions and zoom gear (for the zoom lens you want to use) and either a dome or flat port (macro) at the end.
For compact/fixed lens bodies then there are cheaper options as long as you are happy with the image quality/AF etc. I had different compacts for probably a decade until it got to the point that a new compact body plus housing was about the same cost as a housing for my main camera.
Pros/Cons: Much more expensive/larger/heavier housings needed for DLSR/MILC but can mix/match with different lenses.
Plus other bits like: vacuum pumps, trim weight/float system, handles/tray/arms, strobes, sync cords.
Hard to sell second hand if you upgrade although certain bits can be upgraded depending on the OEM placement of buttons and physical dimensions. Ikelite have a different "back" for the R5ii vs R5 housing for USD850 although the power switch needs to be on before closing it. A new housing is USD2000.
Ikelite are at the cheaper end of the spectrum. Aluminium housings start at about double the Ikelite cost and a Nauticam setup for a R5ii +RF14-35mm would be about USD10k (plus the camera/lens

)
Nauticam do have a zoom gear that can use the control ring for the EF-R adapter but otherwise not.