1. Clear a flat area on the sea bottom.GAH. I meant underwater. Not underground. =(
And this is for 1.6.2. So no mods outside of IC2.
2. Build a wooden platform above the ocean surface.
3. Build a sand sculpture of the interior space of your underwater base.
4. Set fire to the wooden platform, or just cut away with an axe. Mining Laser might work.
5. Cover the sand on the sea floor with glass.
6. Remove the sand.