Literally, the creatures in the sea, the ones having life. Life is ψυχὰς [psychas] , which has many different shades of meaning, but here describes life on earth in its external, physical aspects. (breath of) life, life-principle, soul, of animals.1 The sea is to be taken literally because both living creatures and ships are destroyedneither of which have biblical precedence if the sea is taken as a symbol representing the Gentile nations. See commentary on Revelation 7:1 . When Isaiah prophesied of this time, he mentioned that ships would be among those things which God destroyed as He brought down mans pride (Isa. Isa. 2:16).
The impact will also generate unimaginably huge tsunamis (tidal waves). Those giant waves will destroy a third of the ships on the worlds oceans, capsizing huge ocean-going vessels and completely swamping ports. The resulting disruption of commerce and transportation will cause economic chaos.2
1 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: University of Chicago Press, 2000), 893.
2 John MacArthur, Revelation 1-11 : The MacArthur New Testament Commentary (Chicago, IL: Moody Press, 1999), Rev. 8:9.