Did i find the impossible? Did STP make a mistake in Making Money?

Discworld is so well researched. By now, I just take it as given, that information in DW books is accurate. But there is this passage in Making Money, which gives me trouble.

When >! the 4000 golems march under water !< the book states, that sound travels poorly in salt water. But doesn't sound travel very well in the sea? Whale song and sonar and whatnot?

It's not a big deal, even master wordsmiths are only human. Made me go "huh, that's strange" is all.