I can't comment on the adslider (it's a bar with a wheel on you roll on the floor) - since I don't have one,
- but I didn't once know a swimmer who used to complain about the ache after using one
However I know these work:
- those abcradle things
- a kegal exercise where you just lie on your back with knees up and move your hips up and down
- lie on back and raise legs, or kick either one at a time
- those things that electricute you to tense the muscles, most useful for learning to isolate the muscles
But look out. If you want abs you need to be very slim, medically under weight I hear most of the time. If you're normal weight it does show but it's not really proper abs