Can TimberTech's Floorizon Planks be made into a watertight surface?

No. TimberTech Floorizon is made with weep holes every 9" on the tongue. These weep holes allow water to drain off of the deck. Correct installation of the product with hand pressure gives you approximately a 3/16” gap between the tip of the tongue and the back of the groove of the preceding starter strip or plank. This gap allows the product to expand and contract with temperature changes. If this gap is made watertight by a caulk product, the product will not have the ability to expand and contract in the gap. If your project requires a dry space beneath the deck you can install TimberTech DrySpace.  more