Remember that whether something floats in water depends on its density. If the density is lower than that of water, it will float. If it is higher, it will sink (or sediment). If your bagel sinks, it means its density is quite high, it's dense. You can resolve this by leaving it to proof for longer. The yeast in the dough will create more gas bubbles. Gas has a very low density, so will lower the density of the bagel as a whole!