The short answer is, all concrete cures under water without the need for you to do anything special. However, you can purchase a special type of cement at your local home improvement center or hardware store known as hydraulic cement. This type of cement has extra calcium added to it which speeds up the drying process--believe it or not, this formula was actually discovered by the ancient Romans, who used volcanic ash in their underwater concrete.
Build the form that the cement will be poured into and anchor this form under water where you wish your concrete.
Mix your hydraulic concrete as per the manufacturer's instructions--this is generally 1 part cement to 2 parts sand to 4 parts gravel. Add enough water to make a thick paste, just slightly thicker than pancake batter.
Pour your hydraulic concrete into your underwater form rather quickly after mixing, as the hydraulic form of cement sets up more quickly than normal cement.
Protect your poured concrete from strong waves or currents as much as possible. Allow the concrete to cure for 48 hours before removing the form. Allow to cure for 7 days before placing your concrete under stress.