This is specific to Seacam, because their strobes have TTL circuitry inside the strobe - that's why they have Canon- and Nikon-specific models. This is also why they need sync cords to operate properly - they need bidirectional communications with camera's hot shoe. The only other strobe that does this that I'm aware of is ONE UW 160x, which is closely patterned after Seacam. With everything else, your options with a 5D IV are:
Simple wired connection from the camera hot shoe to the strobe(s) - this will give you triggering, but not TTL, as there is nothing to tell the camera that a TTL-capable flash is present, or translate its protocol into a form usable by the strobes. You will need to use knobs on the strobes to adjust power.
Simple (non-TTL) LED trigger, optical bulkhead and fiber optic cables. Same as above, except the cables are not as bulky and don't penetrate the housing, so there's less potential for water ingress. You can also detach and attach them in the water if needed.
TTL-capable converter (take your pick) with sync cords and Ikelite (for DS strobes) or Nikonos (for everything else) bulkheads and sync cords. This will give you manual and TTL options, but sync cords are bulky, expensive, cannot be detached in the water and can leak.
The same TTL converter as above, but with optical bulkheads and fiber optic cables. This will give you the same TTL and manual options as sync cords, but with thinner, less expensive cables and no risk of water leaking into a connector.
If you want to run high-speed sync (faster than 1/200s on 5D IV), you will need either the new Seacam 160D strobes and sync cords, or the new Retra Prime/Pro with an HSS-capable converter and fiber optics (Retras do not have electric sync capability at all). Keep in mind that high-speed sync significantly reduces strobe brightness, as instead of emitting a continuous pulse, HSS makes it flicker at very high speed.