Density is a property of a material that measures its mass per unit volume it occupies. Water has a density of 1 g/cm^3 and anything that has a density higher than that of water will sink. Group 1 elements are also called alkali metals. Their names, formula, and densities(in `(g)/(cm^(3))` ) are as follows:
Lithium, Li, 0.534
Sodium, Na, 0.968
Potassium, K, 0.89
Rubidium, Rb, 1.532
Caesium, Cs, 1.93
Francium, Fr, 1.87*
From the criteria of buoyancy, only elements Rubidium, Caesium and Francium will sink in water.
*The density of francium is not yet fully understood due to some nuclear issues.
**Placing Alkali metal on water are extremely dangerous. Their reactions are explosive.