All of the dolls can be gently rinsed in water or washed in warm soapy water, even the dresses.
For the fairies and angels : When you have washed them stand them on their heads with their feet resting up against a wall or container so the dress can dry in place.
However, any feathers will need to stay away from water as they will never be the same again, you will just need to protect them by wrapping them in something water-resistant.
For the mermaids: just rinse them, but if little bits are stuck under the glitter a gentle toothbrush or paint brush wont hurt at all.
And they are not meant for bath play or fish tanks.