This guy cracks me up. Let's look at his "hypothesis" . . .
1. It would have to be bright (the relatively tiny human eyes).
2. It would have to be wet (the aquatic adaptations) and
3. It would have to be safe, both from sea monsters and from cosmic radiation.
1. The average distance of Jupiter from our star is more than 480 million miles. The average distance of the Earth from our star is 92+ million miles, an astronomical unit. So Jupiter is more than five AU from the star. Yet we are to believe that it was "brighter" there 50,000 to 100,000 years ago. Yeah . . . right . . . pull the other one. Don't try to feed anyone that "Purple Dawn Age" bullsh*t--it's right up there with the Illuminati and ancient astronauts.
2. Ganymede certainly is wet . . . beneath it's mantle of water ice. It is reasonably estimated by astronomers that from 45% to 50% of the water on Ganymede is frozen. That which is liquid in wrapped around the (probably) molten iron core. It wouldn't be swimming weather on Ganymede.
3. Ganymede undoubtedly is safe from sea monsters. What atmosphere there is has a surface pressure on one microbar--that's one one millionth of the atmospheric pressure on our planet at mean sea level. Apart from making breathing difficult, it's not going to provide any protection from cosmic radiation. However, Ganymede does have a magnetic field of its own--not that that matters much, since it is within the magnetosphere of Jupiter. However, as Wikipedia
points out: The radiation level at the surface of Ganymede is considerably lower than at Europa, being 50-80 mSv (5-8 rem) per day, an amount that would cause severe illness or death in human beings exposed for two months.
Not that any of that matters, given the lack of an atmosphere or any drinkable water.
Basically, Gunga is more likely to believe something the more improbable it is.