Since it seems that old Skyrim community is still very active, I am retro-porting some of my SE mods to classic Skyrim, however, I can't test them because I don't have old game installed, and some mods will require a bit of users work to complete porting job. I'll include specific instructions if necessary, and tell me if you find issues.

DESCRIPTION

I was trying to do a patch for Coco's mermaid outfit (for testing some swimming animations), and ended up creating a mermaid-transformation mod.
You can craft mermaid ring in the forge, characters wearing this ring will undress and autoequip mermaid outfit when swimming, and remove all items when they come out of water, after a little delay (so, you can redress player character if you don't want to get naked). I've added extra effects like water breathing, faster swimming speed, etc, as improvement, and if you use DAR framework, you can get custom animations.

INSTALLATION

Install Coco's mod or at least meshes and textures, build models for your body with bodyslide, etc.

Install Armor Clothes and Weapon Framework, it is necessary for keywords that will force custom animations, and other features.

These are the mandatory requirements, but, since the main purpose of this mod is to swim like a mermaid, you will also need:


Specific instructions:

This mod was created to work with Open Animation Replacer, but since it isn't available for original Skyrim game, you can use Dynamic Animation Replacer instead:

-Install DAR and requirements

-Go to Skyrim SE page and search Walk With Elegance mod, then, download fins addon manually.

-Extract it, and add desired custom animations to folder 70010 (you can rename folder if you have conflicts), I use Gunslicer's swimming animations, not sure if they are available for classic Skyrim, or maybe exist similar aminations.

-Install manually on Skyrim directory.


TWEAKS:

Instead of uploading a lot of different versions to cover each taste, I'll explain how to modify it to adapt things to your own preferences.
Note: it's recommended to make a backup of the pluging to restore it easily if you have issues, without need of reinstalling full mod.

-Edit delay time: if you want to modify post swimming delay time, open YSLAF.esp plugin with SSEEdit or Creation Kit and search magic effect
called YSLAF_EnchEffect and go to attached script, then set RecoverTime variable to desired value (5 seconds by default).

-Enable/Disable auto redressing: You can enable or disable a bool variable called AutoEquip in the script attached to YSLAF_EnchEffect, use SSEEdit or CK
to set it to true if you want player character to auto equip original outfit when mermaid effect ends, or set to false if you prefer to equip items manually. Note that weapons are equiped first (when you exit water) to help you if you are under attack.

TROUBLESHOOTING

Incompatibilities:

It can cause conflicts with mods that add swimming features (undress character when swimming, equip swimsuits, etc), but you can try to solve
it disabling these features, or as alternative using mermaid outfit as swimsuit for these mods.

For example, I've tested with BOSSE, and it can work if you set swimming nudity option to full nude for player, you can also set armor slots used by mermaid items as no removable in BOSSE settings. I haven't tested with NPCs, but since BOSSE can identify AC&WF keywords, I suppose it should work. As alternative you can try to add mermaid outfit as a swimming item and let BOSSE underwear system to deal with it.


Male support?

Male characters can use the ring, but outfit part is disabled because Coco's outfit is for female only, but if you know a similar armor for male, I can try to
add male support.


Why you have removed armor flags from items?

Well, there are some mods that apply penalties to swimming if character is wearing armor or clothes, a mermaid that swims slowly has no sense...
Other mods that force custom animations depending on equipped armor can cause issues too, so, I think the best way is to remove these flags and
keywords. You will lose protection and armor perks, but probably you don't need them when swimming.


Other issues?

Tell me if you find a bug, and I'll try to fix.

Credits

This mod only includes a plugin and a script, so doesn't contain assets or resources from other mods, only references, however, it will not be
possible without them:

Mermaid outfit by Coco.

Dynamic Animation Replacer by Felisky384.