Swimming molluscs

Semi-serious question really. I found some self sown seedlings of a plant I have never managed to get seeds from. They always seem to go long before I think about collecting them. So I dug up as many of the seedlings as I thought I would want. Potted them up and......................almost all of them got eaten. Mollusc trails on them so know the culprits.
So I moved the survivors into the Alpine house where there are fewer pests..........They got eaten.
So the three last ones I put in a seed tray, stood that on an upturned pot in a watering tray full of water....................they have been eaten.
Went back to the original plant and there are still a few self sown seedlings under it............completely untouched by anything teeth of any sort.
So why? And how did the molluscs get across the water? And how do you good folks keep the pests at bay.

 

Sounds like they are in the pot itself, try soaking it for a bit, they don't like that.

My outside ashtray filled with rainwater, I see a snail commited suicide in it overnight, must have had a hell of a nicotein hit.

 

First thing I checked was the pot, I really dislike these types with a well underneath. Turfed the plants out and looked in the soil too, no molluscs in there either.
Still would love to know why they eat the potted ones and leave the others alone though.