If you intend running a cable outside, you will need specialist outdoor Ethernet cable. It’s not so much the water that damages the cable,more the sunlight. Over time the UV rays dry the cable causing it to crack, this then allows water ingress. The image below shows outdoor Ethernet cable on a bulk roll.
Outdoor cable is available in Cat5e and Cat6, the image below shows a ready assembled Cat6 outdoor Ethernet cable. Expect to pay around £27 for a 50m cable. You will struggle to find it on the high street, but they are readily available online.