Re: will 5200 cure underwater?
I might offer a little different perspective...
It is my understanding that 5200 cure is isocyanate based - water reacts with isocyanate groups to form amines, amines 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, isocyanate groups are reacting with groups on the surfaces that the caulk 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...
will it work if you put it right in water? perhaps, probably, etc...??? depends on the specific application and so forth. is it going to do much better if the instructions are followed? very likey so.
imho...