From no prior research, just off the top of my head, I would say that 2.5in SSD's are equivalent in liquid resistance to 2.5in HDDs. They are both protected by the metal casing but if water were to get inside, game over for both (the SSD because it would short and the HDD because it wouldn't be able to read or write data through liquid). A SSD like the Macbook Air uses is probably less resistant to liquid damage simply because there's nothing protecting it, if water makes contact it would be the same as it getting to the inside of a 2.5in SSD.
That's my hypothesis at-least...