Behold, the hope of him is in vain - That is, the hope of taking him is vain.
Shall not one be cast down even at the sight of him? - So formidable is his appearance, that the courage of him who would attack him is daunted, and his resolution fails. This agrees well also with the crocodile. There is perhaps scarcely any animal whose appearance would be more likely to deter one from attacking him.

Behold, the hope - If thou miss thy first advantage, there is no hope afterwards: the very sight of this terrible monster would dissipate thy spirit, if thou hadst not a positive advantage against his life, or a place of sure retreat to save thine own.

Behold, (p) the hope of him is in vain: shall not [one] be cast down even at the sight of him?
(p) That is, that trusts to take him.

Behold, the hope of him is in vain,.... Of getting the mastery over him, or of taking him; and yet both crocodiles and whales have been taken; nor is the taking of them to be despaired of; but it seems the "orca", or the whale with many teeth, has never been taken and killed (o);
shall not one be cast down even at the sight of him? the sight of a whale is terrible to mariners, lest their ships should be overturned by it; and some have been so frightened at the sight of a crocodile as to lose their senses: and we read of one that was greatly terrified at seeing the shadow of one; and the creature before mentioned is supposed to be much more terrible (p).
(o) Vid. Scheuchzer. Physic. Sacr. vol. 4. p. 846. (p) Scheuchzer. ib.

the hope--of taking him.
cast down--with fear "at the (mere) sight of him."

Hope - The hope of taking or conquering him.