[yk]

In a single event, the Roman navy during the first Punic war. Rome lost the entire invasion fleet with estimates of more than 90.000 drowned during a storm. (It is still the second worst maritime disaster in history as Wikipedia counts. [0] Wikipedia lists the Mongol invasion of Japan as the worst, with more than 100.000 lives lost.)

The bloodiest war in Roman history was the second Punic war, that is the battles lost against Hannibal.

[0] https://en.wikipedia.org/wiki/List_of_maritime_disasters

[EGreg]

What about the sinking of the Spanish armada invading Britain? As far as maritime disasters it was pretty huge. And that seems to have turned the tide of history.

[_delirium]

That's third on the list linked above, with estimated 15-20k men lost.