There are two dominant species on the planet Someplace. One is humanoid, much like us. They have modern-day technology, and have similar physical traits. There are around 1 million people in the military who actually fight. Military distribution is like the US military.
However, the other species is a sea-dwelling species, with similar technology, but adapted to water. They have some way to grab things (think webbed hands or feet). They cannot survive on land, needing moisture for their skin (gills?) and can only breathe in water. They have a similar size to the humanoids, and have a similar number of military personnel. They do have a limited "air force", with planes filled with water tanks, and such. They have a lot of boats filled with water.
Spies and intelligence don't exist, because the two species look quite different. They can communicate, but they speak different native languages.
So, my question is, could the sea-dwelling species attack the land beings, and possibly win? If so, how could they win? If not, are there any modifications you could make to the sea-dwellers to give them a chance to win?
Edit: I know there's a similar question, but it's about humans attacking sea creatures, not the other way around.