The float test is to test whether there is enough air in the egg to float the weight of the body and liquid in the egg.
At day 17, I would expect it to sink, since the air cell is not yet big enough to hold it up above the surface, and you still have liquid in the egg at that stage. Also remember that an egg has pores just like your skin, it isn't solid, it allows air (and humidity) to pass through it.
The float test is best used as a last resort on eggs that you suspect are dead, beyond day 24, since it wipes off the bloom and if there were an internal pip at that time, the chick could drown.