Can u help me understand this poem and some words in it?

Taken means, if they "depart"(on take a chance in this "tide") they will be lead to fortune. Shallows means that it will be times when the sea is thin, and if you know anything about boats, they don't travel well or far in shallow water. Basically this whole poem is saying take chances when they are present, don't chase after them once they are gone.