That's a pretty far reach from the magical background - which I assume a general knowledge about magic. Still I consider the core rulebook to be pretty common (read: presented in trid shows) knowledge. If it's naga, it would have a colorfull scales, and would be awakened. Which you can't tell right now. (it's too dark to judge colors, and visual feed doesn't tell anything about it being awaken)
On the visual side, it could be any type of large snake. (but large is also questionable, because most of it is probably hidden in the water stream, and that is not penetrated by Sam's radar).