All Epoxies will cure underwater. They use a chemical reaction rather than oxygen as the catalyst. However, they will also cure slower underwater because of the temperature difference AND they might wash out if used before they are fully cured.
As far as 5200 goes, it will cure underwater. It's designed to do so, and in fact it needs moisture to cure, just as well as air dry. As far as speeding the curing process up with water, you won't notice any difference. It will not stick underwater though. It needs a dry surface for a proper bond. Waiting 24hrs. is ok, but 48hrs. is best for 5200.
From someone with far more chemical experience than me:
It is my understanding that 5200 cure is isocyanate based - water reacts with isocyanate groups to form amines, which react with other isocyanates to form ureas linking the molecules together and increasing the the strength, water and solvent resistance each time a new bond forms.
In addition, the isocyanate groups are reacting with groups on the surfaces that the product is in contact with, chemically bonding it to the surface and covering polar groups that would let water wick between the sealer and surface it is being applied to. If there is too much water (as in immersed in water), a large portion of the isocyanates are converted to amines and do not have time to react thorooughly as described above - hence the reason for the delay before putting it in water. For maximum adhesion, strength, water and solvent resistance, you want slow cure from airborne moisture...
