The tide must be taken when it comes

Meaning: We should use all opportunities when they come, before it's too late.

Note: (the) tide (noun) = the rising and falling of the sea caused by the attraction of the moon and sun | Compare: "There is a tide in the affairs of men, Which taken at the flood, leads on to fortune..." (from "Julius Caesar" by William Shakespeare 1564-1616)

Origin: Ships (especially in the old days) often need a high tide to leave or enter port. Man cannot control the tide, so when it comes ships have to use the opportunity it gives them without delay.