About designing environments, here's my rules (actually, the rules are about preparing in general):

1 - remember it's a game, so design for your players.
2 - don't prepare more than you need.
3 - whenever you design something, design a secret.

Rule 1 reminds me that a setting might be wonderful to me, but it needs to be played in. That means I need to leave room for the players to make their mark. Maybe a player would like to introduce a certain religion, or wants to put their mark on underwater farming, who knows, players are unpredictable. Leave them gaps that you can fill with adventure.

Rule 2 is almost an extension, but it's more practical. I have practised various approaches to preparation over the years, and most of those have left stacks of notes unused. But once you set a part of the setting in concrete, on paper or only in your head, you can get fixated on it. In the beginning of the campaign, make sure you give your players time to breathe it in a bit. It could give you the feedback to finetune your idea before the concrete sets.

Rule 3 is something I think I saw in a magazine somewhere, and it works well. If you want to make sure you remember (for instance) your innkeeper, give him a little story. He could be quite secretive because he has a son in the basement who is a werecreature of some sort, or maybe that is in the past. This little story doesn't automatically create more adventure, but whenever you need that npc, your mind often 'snaps' back to the story behind him. Knowing his background often helps you remember what voice you used, or other little details. Human brains suck at remembering details unless we have a little stoy to link them. This method works for me in memorizing npc's, places, artefacts, religions, and for most other things.

By now I realise that I have never answered your question about the environment. Silly me.

If I would do this (so feel free to ignore), I would suggest placing the characters in a scene before their creation. For instance, several coral fields have recently been laid bare in this outpost the characters are in, so that the shepherds of sillyfish herds have been forced to take them lower, to darker areas with more predators.
This little scene gives loads of options without being specific. Your players can decide how their characters got there, or they can pick skills to hunt sillyfish-eating predators. Maybe they're from a line of shepherds themselves. They can immediately place them into the environment (rule 1), you can have instant adventures examining the bare coral, or protection the herds, without you needing loads of background (rule 2), and only you know that the darkshark king has bribed an npc to destroy the fields, in order to lure the sillyfish to darker waters.

Ok, maybe the names are too silly. :P

And maybe I still haven't answered your question. Please let us know if this is what you were asking. :)