what light does the opening scene throw on the danger that the sea could pose to ships ?

Antonio is worried that he will lose everything if his ships do not return safely to port. This alludes to the fact that when the ships are sailing through dangerous waters and weather, their fates are uncertain.

"our mind is tossing on the ocean,

There, where your argosies with portly sail,

Like signors and rich burghers on the flood—

Or, as it were, the pageants of the sea—

Do overpeer the petty traffickers

That curtsy to them, do them reverence

As they fly by them with their woven wings."