Sil is waterproof but does not have much of a resistance to water pressure. Condura is able to better resist water pressure.
This is a great article
about dry bags and some info on sil that might surprise some of you.
Quote from above article:
As a point of comparison, consider that even the lightest of the commercially available polyurethane-coated nylon gear sacks have hydrostatic head values that usually range from 5,000mm to 10,000mm (note also that heavier-duty dry sacks intended for paddling use typically have much higher values). In contrast, the hydrostatic head of standard weight silnylon, according to most manufacturers' specifications, is only 1 to 2 pounds per square inch, which translates (for consistency with industry conventions) into a range of 700mm to 1,400mm. In other words, standard weight silnylon, on average, is less than 15% as water resistant as even the lightest of the conventional coated nylons.