Most of water blocks (for CPU and GPU) are made using nickel plated copper for bottom part and acrylic top part.

It looks nice, as allows to put more RGB leds :-) Plus it is also significantly cheaper and faster to make.

But all else is bad. Thing is that acrylic has quite high thermal expansion, and it is much higher (4-5x times!) compared to copper.
Temperature of water block parts constantly fluctuate and with different thermal extension sealing places are constantly wearing out.
Add to this very fragile threads that crack due to any excessing tightening.

Proper water block must have double sealing and also must have same metal top as the bottom part.
Yes, it won't be so pretty - but it will live much longer and won't endanger your expensive computer parts.