The threat of drowning, both literally and figuratively, runs throughout the novel as a motif. It appears first in Chapter 1, when David recounts being born with a caul (a fetal membrane over his face), which was raffled off:

The caul was won, I recollect, by an old lady [...]. It is a fact which will be long remembered as remarkable down there, that she was never drowned, but died triumphantly in bed, at ninety-two.

It may seem strange to modern readers that this woman would want to buy David's caul, but it is because a caul had superstitious significance in 19th-century England. Being born with a caul was said to mean that a child would never drown. The woman who buys the caul, David admits, doesn't ever go to sea and wouldn't have had much chance of drowning anyway. This joke allows David to play with the idea that he may have been born lucky without actually admitting that he believes in such things. From the beginning, luck is somewhat up in the air for David. He meets a mix of good and bad fortune that is partly determined by his circumstances and partly determined by his own actions. Ultimately, he seems more lucky than other characters. His caul does not necessarily predict this good luck, but it does foreshadow it.

Still, maybe the caul foreshadows misfortune with regard to drowning. Drowning holds an outsized place in David's life, even if he remains safe from it in the direct sense. Drowning is the deepest fear of Little Em'ly. She fears drowning because that is how her father died. Although she doesn't die this way, her fear turns out to be founded because Ham and Steerforth do drown. Even David is not wholly protected from the threat of drowning, at least in the figurative sense. He describes in Chapter 33 being "steeped in Dora":

If I may so express it, I was steeped in Dora. I was not merely over head and ears in love with her, but I was saturated through and through. Enough love might have been wrung out of me, metaphorically speaking, to drown anybody in; and yet there would have remained enough within me, and all over me, to pervade my entire existence.

The idea that David's love for Dora would more than drown anyone else again suggests that David is surrounded by the threat of being inundated. But it also reveals how unlikely his eventual happy ending is. Through a combination of luck, determined hard work, and good people surrounding him, David manages to tread water in a world where most people go under.