I have a PoE camera and the wiring is about 3 feet too short. The Ethernet that was pre-wired is about 100 feet and it isn't practical to redo the wiring.
How should I extend the cable? The connection may be exposed to a little rainfall, so a little water resistance would be needed
Not what I'd do, but there's no such thing as a cable that isn't practical to redo in my world.
Waterproof ethernet coupler. They are commonly available. Assuming the case (unstated) is that you have a plug, it's a female to female coupler and a water-resistant housing that goes over the coupler, enclosing a short section of the cables leading to the coupler.
You'll also need an exterior-rated (water and sunlight resistant) patch cord.
Extend the wire while indoors with a coupler, and make sure that the section exposed to the outside is weather-proof.
There are also weather-proof couplers available, but obviously at a higher cost:
If you really don't care to re-use the coupler, I'm sure you can seal it up with any assortment of sealant or material available.
Was it solid core or stranded core cable installed ? Solid is cheaper and easier to work with, but tends to break after a few months if subjected to movement so its ideal for inside walls and roofs.
If you have solid, I'd terminate it in a waterproof jack, and then use a stranded patch cord to go to the powered item. The stranded will be replaceable in the future.
If you have stranded cable running the whole distance, then I'd do the same BUT you will either need a jack designed to take stranded wire in its punchdown connections. Don't try and ferrule or solder all the strands together, that never works reliably.
If you can't find a waterproof jack. then shorten your 100 foot run so the joint will be in shelter. Also make a point to put the joint higher than the cables approaching the joint, so rainwater will drip off and not run along the cable into the joint.
Finally you can stop this kind of thing in the future by improving your method. Pull the cable(s) from the box(es) and through the building. Then terminate the far end. Leave a little slack cable in case you want to move things later.
When that's done, work your way back along the run securing the cable loosely. Don't use staples - I personally like cable/zip ties but formed into a wide loop or sling. There are few times where strapping cables together is a good idea.
Then and only then do you cut the cable to length. Again allow yourself a metre of slack. Many pro installers will bring cable into a cabinet in a nice loop, so the cabinet can be rolled/opened without undue strain.
Then terminate the patchpanel end into a jack, and test the run with a simple wiring tester.
