Depends on the CCTV. Any number of home security cameras like Wyze, Eufy, Nest, etc. can stream live video to a PC or phone. I have installations of all three mentioned, and they can all stream to my phone, and a couple can stream to my PC

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

The cameras are low-res analog.

And that's a problem because?

https://www.amazon.com/analog-ip-camera-converter/...

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

Tablets, AFAIK, don't have plug-in ethernet. Many of the devices shown are POE adapters, others are video-over-cat5/6 adapters, using co-ax at each end. None of them will connect to a tablet input (tablet displaying the camera image).

That's a separate problem, isn't it? Wired ethernet can be connected to a Wi-Fi router and a tablet can connect to the same Wi-Fi as well.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

It seems to me that you are bending over backwards to keep crappy cameras to display on a high-res display, which seems a bit pointless.

https://wyze.com/wyze-cam-pan.html $39.99+tax actual high res and runs over Wi-Fi.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

Substantially because they are low-light cameras with zoom and aperture control. They are for rear-view on a large boat.
KISS applies too. Electronics in a marine environment.

You've made choices that box yourself in; unless your tablet has an HDMI input, you will need to have hardware and an application that can take streaming data and properly display it on your tablet.

If I had to do it myself, it probably could be done, but it would cost me too much time to do it for free.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

Have a look for IP webcam. It is wireless but there is nothing stopping you having a wireless switch to create a LAN on your boat. I've done it using old windows 8 phones in my garden.

The tablet is wifi so you have to convert the camera to a format that can be accessed over wifi. You have the answer, I'm not sure what else you're wanting.

I wouldn't be surprised that there are no cameras suitable for boat use that you can connect a tablet to directly. WiFi cameras connect to a router generally. I think for your described use I'd want to avoid wifi as much as possible anyways.