Buy an 8' long piece of angle or channel iron, or all-thread rod. Have someone position and hold the end against the concrete, while a second person uses a small sledge hammer on the end that juts out above the water level to drive the iron rod into the concrete chunk.

Pre-arrange a method to signal the top-side person so you can control or stop the hammering in the event of a problem or the need to reposition the iron rod.

And keep your fingers clear.