In many cephalaspid bubble shells the true foot is lost or very reduced, and they have a secondary foot developed from the posterior mantle flap. Some sacoglossans have a transverse groove across the foot just behind the head, suggesting to me that the part of the foot behind the groove is also a secondary foot. One side-effect of producing a mucous trail is that nudibranchs, and other sea slugs advertise their whereabouts at all times. Carnivores such as Navanax inermis and species of Roboastra rely on this tell-tale trail to find their prey. It is also apparently useful in the peculiar 'trailing' behaviour of species of Risbecia .
Some sea slugs swim.
This behaviour has evolved independently many times within the opisthobranchs. When I get a chance I will prepare a separate page on swimming. Dear Valda, It's one of the gaps in the General Topics list that I have been planning to fill. I have prepared a Locomotion Page . I hope that answers your question.