What Type of Caulking will adhere to Composite Decking for water proofing?
Typically you need those gaps for expansion of the boards in hot weather- if they're tight to each other they can get damaged as they expand from heat.
They make under-deck panels that catch the water and direct it out a side that work very well. That would be a better solution than caulking, which would most likely need to be re-done every year or two anyway.