Eternal Winter is a small mod that enables snow in every biome.
By default, snow is disabled in oceans and rivers, so they don't freeze.
You can change this in the EternalWinter config, though.
When generating new chunks with this mod, the chunks will be covered in snow
When loading an existing world, snow will build up when the downfall starts, but it will not retro-generate the snow
The project icon is public domain and taken from here