Poor mermaid. No mermaid, no cry.....Wait! What exactly is she crying? Salty air bubbles?
That net must be one of those magical bottomless ones. To capture 100 merman and loli scyllas and it looks so small from the outside. Or could it be, he didn't capture all of them in one swoop? Oh noes! Than the invasion isn't actually over and they shouldn't relax.
You could theoretically see the difference if there's a significant difference in salinity. Maybe the sea is really salty and her tears are pure water. Ionno. Or maybe this is just a manga and it's magical tears.
Everything about that net was off, like how he cast it underwater, how it closed on the enemy, and what it's hanging from (how it hangs at all underwater)...I suppose if he can make a barrier that extracts breathable air from water, then he can probably "Green Lantern" up some kind of barrier netting....