Went down does not necessarily mean that Tobias went down a steep bank to the river. Obviously the water level is below the land, but it need not be much below.

Suddenly is not in the Greek, but fish do not ordinarily jump out of water gradually.

A large fish leaped up from the water and tried to swallow the young man’s foot: This is a difficult picture if the reader is thinking of Tobias wading. He was evidently sitting on the bank, probably splashing his feet in and out of the water. Apparently the fish was attracted to the movement, and jumped up to grab what was splashing in the water. But the fish was not just trying to bite the foot; it wanted to eat it. The fish thinks it is an animal of some kind and wants to swallow it. This has a bearing on the translation large fish or “huge fish.” The Greek only says it is big—big enough that it thinks it can catch in its mouth and swallow an object as large as Tobias’s foot, and big enough, when caught, to provide more than one meal (6.6).

And he cried out: In a number of languages, particularly in Africa, a vocal expression called an “ideophone” would be natural and appropriate here. Ideophones do not normally fit into the usual grammatical patterns of a language, yet they express such things as an emotion, a quality, or a movement, and may sometimes mark or emphasize a feature of discourse. In English, an equivalent of an ideophone in this context might be “ ‘Ouch,’ he cried.”

Quoted with permission from Bullard, Roger A. and Hatton, Howard A. A Handbook on Tobit. (UBS Helps for Translators). New York: UBS, 2001. For this and other handbooks for translators see here.