When you submerge an object into water (or any other liquid), it displaces a given volume of liquid. The upthrust experienced by the object is equal to the mass of the liquid displaced. (ie. the volume of the liquid displaced, multiplied by its density.)
The resultant force on a submerged object (assuming no external force is applied) would hence be the upthrust less the weight of the object. If the object weighs more than the liquid it is displacing, it will sink. If it weighs less, it will float! Simple really!