Today I've been looking at two older things I wrote some while bacl, which summed up my extensive thoughts on forests as an agressive force and the sea as an alien realm. The Green Hell and the Circle of Life and Death and The Deep Blue Under.

I think they still are both great approaches to give this setting some spice and unique style.

The first idea is that the world has an overaboundance of life. Things grow and spread, and it's impossible to rein them in. The fundamental processes of life are feeding and reproduction, neither of which is pretty or pleasant. Aside from some bacteria, all living things need to consume dead matter to stay alive and produce new life. You don't have to kill it yourself, but for you to live, something else must die. Our culture treats growth as amazing and wonderful, but when you look at all the things that make up life, it is also terrifying and revolting. Not least because it also tells us that there are many things that are waiting for us to die and to consume us, so that they may live and reproduce.
If this can be translated into monsters and phenomenons, I think it would be an amazing thing to make nature intimidating.

The other idea is that below the surface of the water is very much another world, separate from our own. A world with little light, barely any gravity or warmth, and no air at all. A world that exists in three dimensions instead of two. Home to creatures that are much more unlike us than other mammals, reptiles, amohibians, and even birds. Many of which get much larger than anything on land or in the sky. We can swim on the surface and take peaks into it that fade into a dark blue haze almost immediately. But we can not really enter the extend of this world and everything that passes below the surface will be gone forever. What goes up must always come down. But what goes down will always stay down below. The sea is part of the Realm Beneath and Beyond, just like the underground world and space. Within that context, travelling across the sea in ships should be a strange and frightening experience. You are trapped on a fragile construction that is the only thing that keeps you from being pulled into a hostile alien world forever. Surface fish are still just animals, like the ones that live in rivers and lakes (which are also ambigous places), but the creatures of the deep sea are effectively demons.
I think it also makes isolated islands great candidates for being very strange places. They are different worlds from the familiar world of the mainland. They look like land, but they are rising out of the sea. You clearly can not trust them.
I definitely want to include a race of fish men. Like Deep Ones, kuo-toa, and murlocs. And they would need to be really alien and not trading or cooperating with surface peoples. They are inteligent and have a culture, seen using spears, knives, nets, and other tools, and having priests and unfathomable religions. But surface people can not really interact with them as equals. If someone communication with a priest can be established, their motives and behaviors would be as alien and incomprehensible as those of spirits.

These are two aspects that I think should really be expanded on extensively, and I'd be happy about any thoughts and suggestions you might have to share. The numbers indiscate that people are still reading here.