The only way clay bricks would stay intact over time in water is if they were completely vitrified by kiln-firing to extremely high temperatures, about 1200-1400 F, which may or may not have been achievable in those times -- and even if it were, the straw would certainly have been destroyed in the process.
1200 to 1400 F is not an extremely high temperature. I've done better than that in my backyard with firewood. Aluminum melts around 1700 F, and I've built fires that turned aluminum cans to puddles.