For tankinis with shorts, you want the bottoms to fit snugly around the hips without being too tight.
The waistband should sit at the natural waistline without being too low or too high.
The leg openings should be wide enough to comfortably cover the buttocks without being too loose or too tight.
The fabric should provide adequate coverage without being too skimpy.
The style should flatter the figure without being too revealing.
If you are especially worried about your suit staying in place when doing things like competitive swimming, water skiing, or a good old-fashioned water slide, look for one-piece suits that are designed to be active swimwear.