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
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I can do absolutely anything. I'm an expert!
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).
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.
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I can do absolutely anything. I'm an expert!
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.