I didn’t even mind the part with the water at the end; I thought it seemed believable enough. It’s not like the aliens were there to settle Earth, they were just an unknown threat present only for a brief time.
Let’s turn the tables around. If a group of humans were to visit a world – a world covered 71% with sulfuric acid, where it rains sulfuric acid, and where the native inhabitants have sulfuric acid on tap – and go down to the surface completely buck-naked with absolutely no protection from the elements whatsoever, would that be believable in the slightest?
That depends on what we needed there. We already go to the moon, with only a suit to protect us, what if it tore, or malfunctioned!?
I can find it believable enough that the aliens needed something from Earth that was worth the risk, or, maybe they had had no exposure to water before/didn’t know it would damage them. I can suspend my disbelief when it comes to these things.