The Sea Horror by Edmond Hamilton

"We were not long left undisturbed in our strange prison. Within a few minutes we saw, approaching our building from outside along the smooth-paved street, a group of the slug-creatures who carried with them what seemed strange suits of flexible metal, with transparent eye-holes. Three of them donned these, fastening them carefully, and then the outer door of our prison rolled down and the three moved or crawled into the vestibule, or space between the doors, which was filled with water, of course. A moment later came the throbbing of pumps again, the vestibule emptied of water and filled with air, and as the inner door rolled down in turn the three crawled into our prison. Their armored suits, we saw, were filled with water to enable them to venture into the unfamiliar element of air, just as a human diver in an air-filled suit will venture into water.

"A moment we humans stared at these strange figures, in sickened horror, while those outside watched us carefully through the clear door, ready to open it and send the destroying floods in upon us at any wrong move on our part. Then one of the three, with a long, slender rod in his grasp, moved to the metal wall of our prison and drew a sketch, or diagram, of sea and land, with slug-like creatures at the sea's bottom and erect, manlike ones upon the land. He pointed from the former to himself, and from the latter to us, and I stepped forward and repeated his gesture to show our understanding.