There are a few things that can make your potato salad watery or mushy. 1. Always start with the right potatoes; use Yukon gold and not something like a russet. Yukon gold potatoes absorb the sauce without becoming mushy. 2. Do NOT salt the potatoes while they are being cooked or while the potatoes are warm. Salt makes potatoes release water, and this can make your potatoes mushy and your potato salad watery. 3. Drain and let the potatoes cool before adding them to the sauce for potato salad. The steam from the hot potatoes can water down your sauce.