GW the % recommended varies depending on who you ask. Usually between 10 and 20%. Sometimes it is given in number per acre. Usually 1 to 3 per acre depending on the size and configuration of the pond and type of fish and goals. As I said earlier most of the data I have seen addresses fish on fish predation and not birds and mammals. Keep in mind that cover both protects fish that can use it but also helps hide the predator. Surface plants are a good example. Fish can hide in it but they also can't see the predators as well. The shoreline is often the biggest structure/cover in ponds so the smaller the pond the bigger % of cover it represents. One point often made is that thick cover such as a pond full of xmas trees greatly reduces fish on fish predation and adversely effects predator fish due to lack of catchable food.

Most people use cover to concentrate fish to catch and not for fish protection. I do use xmas trees for small fish protection adjacent to BG beds. They help protect small BG from LMB and the herons and kingfisher. I remove some of the branches depending on the size of the fish I want to protect and the location. I do agree with your idea that lots of cover will help protect fish from some predators but it will also make it much harder for predator fish to get enough to eat. I have seen underwater films of both otter and cormorants feeding. I think cover would help the fish but both of those predators are very agile underwater and I think would still do to much damage for my liking.

I hope this helps a little and maybe starts a discussion.