Actually, I saw some footage of swimming iguanas lately - and they interestingly flap themselves sideways (as per fish), even though they are basically finless vertebrates, looking quite graceful when they do it.
Perhaps we can invent a new swimming stroke called "iguana style" and add it to the Olympics
As for the Realms, well - the Realms is sort of the ultimate hodge-podge campaign setting... They have all sorts of arquebuses and muskets, as well as the fabled technological island of Lantan, which isn't detailed anywhere
. The gun powder needed for guns is considered magical, though, which is sort of their explanation for how things work...