Even after draining boiled potatoes, there may be water left in them.
One technique to limit soupiness in your salad is to drain the boiled potatoes & briefly return them (without anything else) to the pot in which they cooked.
Then heat, stirring, a minute or so, to help remove (evaporate) water that clings.
Remove potatoes from pot, add dressing to still-hot potatoes (it helps them absorb it), garnish & serve.

I am assuming a mayo based salad. The watery elements of the salad thin out the dressing and the salad becomes soupy over time. If liquefy right away, check if any ingredients are not well drained and make the salad wet. Consider trying a vinaigrette dressing. For a soupy proof salad. I love the mayo potato salad w hard boiled eggs but if it will be sitting out I make the vinaigrette, also delish.