This would be The Kraken Wakes (1953) by John Wyndham.
Quoting from part of the plot summary on Wikipedia:
The novel describes escalating phases of what appears to be an invasion of Earth by aliens
In the first phase, objects from outer space land in the oceans. Mike and Phyllis happen to see five of the "fireballs" falling into the sea...
Phase two of the war starts when ships all over the globe begin to be attacked by unknown weapons and are rapidly sunk, causing havoc to the world economy. Shortly after, the aliens also start "harvesting" the land by sending up biological "sea tanks", which capture humans from coastal settlements, for reasons that are never made clear; the Watsons witness one of these assaults on a Caribbean island.
A point of Stack Exchange etiquette: when posting a question, you’re supposed to indicate what research you’ve done. I recognized the book immediately from your description, having read it myself, but a Google search for "science fiction kraken" brought up the Wikipedia page for this book as the top result. I hope you enjoy rereading the book.