Unfortunately, there's no set pattern to how often you see enemies, or when. The higher your level, the less you will see lower enemies such as the Nymph and the more you will see Daedra Seducers. However, it's all random and because enemies scale with your level, you could still see Nymphs even past level 10. You can also see Daedra Seducers at level 4 (I have). It's just a matter of the frequency changing the higher you go. There's no set cutoff that I can tell you.
The way dungeons tend to work is a "common enemy" is randomly chosen. For example, in one dungeon you may encounter a whole bunch of skeletons. In another, it could be nymphs. Or seducers, more frequently at higher levels. That's just the way the game is... random. If you hear that sixy but deviant female laugh, you'll probably want to run away. There are no mods to keep them clothed. There is a childguard feature that comes with the game that removes some nvde NPCs, but it unfortunately doesn't do anything to the unclothed monsters.
As far as water in main quests, I honestly can't remember ever encountering any but it's been awhile since I've played through the entire thing. So don't quote me on that.
Ok thanks, can you send me a message if one is created so I can get it. I don't want to check UESP so that I don't know exactly how to do the dungeons and I know that you have to go under water with Direnni Tower and Castle Wayrest. Thoguh it might not be a huge problem if Lamia are very uncommon (Almost never, never, never seen would be nice) Also, as far as NPCs what NPCs does the childgaurd clothe? Does it clothe all the Temple Dancers, just some? The tavern woman? The people in the Temple of Kynareth or House of Dibella? Dancers in Witch Covens? (Some don't have dancers thankfully) Could I get a little more detail. Also, when do Nymphs, Lamias, and Seducers normally show up at and yes, thanks for warning me of their laugh but I knew that ahead of time. Also, are there any MQ dungeons that mostly feature Seducers or Nymphs (The MQ dungeons are handplaced). More detail would be appreciated but this is acceptable.