Modern concrete can set underwater, and, at least for concrete where you care about the results, you want to keep the surface wet until the concrete cures appro

Modern concrete can set underwater, and, at least for concrete where you care about the results, you want to keep the surface wet until the concrete cures appropriately. (IANAPE, but I took a grad level concrete course in school) addition/edit: Quick primer. When concrete cures, it's a chemical process that converts free water into an electrostatic gel in the cement crystals. That interlocks the small and large aggregate to make a solid. If concrete dries rather than cures, then that gel doesn't form and you don't get the gel holding it together. If you heat up cured concrete enough, you'll drive out the water and make it a powder again. If you cure something under water, it can technically continue to cure for a very long time. Normally you keep it moist for 24-48 hours, and standard testing is done at 28 days. That will get you something like 90% of the final strength, if it can continue to cure indefinitely. I've tested concrete that was semi-submerged for 40 years where the design strength was 4ksi, and it tested out at 14ksi.