Novel with aquatic aliens that block humans from seeing them

Question

I am trying to remember a novel I read in the 1990s, although the book was probably much older. I don't really recall the main plot, but there was one incident that really stuck in my mind.

The episode I remember had a human spacecraft contacting aliens on a planet they passed near. The buildings of the aliens' city had wide flat roofs cris-crossed by canals. The aliens were moving in the canals, but the humans cannot see them. The aliens allow the humans to see their city, but they block the ship's sensors from seeing the aliens' actual bodies. The humans see only black spots where the aliens are.

Answer 1

The answer came to me. This was a minor episode in Rogue Ship by A. E. van Vogt. However, I am not sure which of the original stories that went into the fix-up this bit was part of (if any).

I had practically forgotten about this book, since I thought it was definitely not the author's best work. The aliens were the only part that stuck with me; they were like a segment from The Voyage of the Space Beagle inserted into an unrelated book.