Here's the answer: When the water reaches about 92% of the height of the block.
If you were filling the bathtub with seawater the block would float when the water reached 89% of the height of the block.
The reason why some glaciers in Antarctica rest on the bottom of the ocean is that the depth of the ocean in that location is less than 89% of the average height of the glacier.
By the way, if you put a 10-lb block of ice in your bathtub and fill the tub to less than the height at which the block floats, then when the block melts the water level will be higher.
That’s not valid. The glaciers have a surrounding volume of water infinitely greater than a bathtub. You would have to ensure the relative container size (bathtub, ocean) are relative to the size of the block of ice (10lb, glacier). The pressure differential is nowhere near the same. Water has weight, so the further you go down, the higher the pressure. This creates a push upward for buoyant objects to the least path of resistance.
It’s about the buoyancy support of the surrounding water. A 10lb block of ice would float in the ocean. It would not in a bath tub because of volume bouyancy support not being equivalent...the pressure differential being orders of magnitude apart.