I am saying I am a licensed/registered engineer just because my opinion is LAW on matters of engineering design.
Sliding doors roll on rails. That is why they are called sliding doors. There is no way rollers on a rail are "water tight". Heavy rain will "briefly" rise the water level in the track "above" the rail height and water "will" enter the "inside" track.
As others have stated, there are provisions for this water to "drain" from the inside track to the outside track.
Sliding doors are not best for water tightness but they have advantage of being a door and not a fixed window (which is water tight.) Also most lanai's are sloped because many methods of enclosure are not completely water tight.
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