I created superflat world with Preset called Overworld, but added snow on top layer as follows:

minecraft:bedrock,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block,minecraft:snow;minecraft:plains;pillager_outpost,village,biome_1,decoration,stronghold,mineshaft,lake,lava_lake,dungeon

Mobs spawn in daylight inside lakes which are created under snow and get out of the lake and attack me. Also when I walk on snow, I can fall thru the snow into lake (including lava lakes), that's now cool.