My first impression was: why so many underwater pictures? Did they figure we were running out of landmass? Or is it because the ocean was one of the frontiers associated with exploration?
Space travel wasn't really considered possible. Air travel was seen as a maybe but the first successful powered flight has not yet happened (or if someone had managed it by then, their achievement had not been widely reported).
The first practical small submarines were around on the early 1800s (man powered at that point) and there had been many advancements in underwater tech (including significant steps in individual breathing apparatus and practical powered sub-marine vehicles) in the late 1800s - so around 1900 the oceans were the next great frontier in the eyes of many of the people who were in a position to care about such things, particularly after the popular success of Jules Verne's "20,000 Leagues" and to a lesser extent other similar works.
This aspiration of the oceans as the next great frontier may also have been driven partly by the massive increase in demand for fuel over that century or two due to the industrial revolution: if you could find a massive source of useful fuel material in that great unexplored wilderness you could name your fortune.
lt;dr: around then, under-water was cool to the public (brought up in the industrial revolution & looking for the next exciting technological developments), potentially very important militarily, and seen as one of mankinds next big steps.
Air travel was seen as a maybe but the first successful powered flight has not yet happened
By 1900 there had been several successful experimental airships, and in 1900 the Luftschiff Zeppelin LZ1 launched. It would be another three years before the Wright Flyer flew though.
Ah, for some reason I had my history backwards and thought successful winged flight attempts came before Zeppelins had seen that much success. Thanks for the correction.
