What does Psalm 18:16 mean?

This continues David's praise for God's rescue during hard times (2 Samuel 22:1). As surely as the Lord drew Moses out of the waters of the Nile (Exodus 2:10), so the Lord drew David out of many waters. David was surrounded by his enemies and felt as doomed as a drowning man, but the Lord pulled him to safety. Earlier in this passage (Psalm 18:4–6), David compared his hardships to torrents of destruction and death. In those moments, he cried out for help and God answered his prayer.

Now David testifies to the fact that God delivered him from "many waters." This continues the theme implied by the word "deliver," translated from a Hebrew term which implies rescue (Psalm 18:2).

Matthew 8:23–27 tells the story about Jesus' terrified disciples and how He rescued them. They were all in a boat on the Sea of Galilee when a powerful storm suddenly whipped up the sea and sent waves crashing into the boat. Fearing for their lives, the disciples woke Jesus and asked Him to save them. They believed they were perishing. Jesus rebuked them for their lack of faith, arose, and, with just a word, calmed the wind and the sea. His power over nature caused the disciples to ask, "What sort of man is this, that even winds and sea obey him?"

We may find ourselves drowning in a sea of disappointments and/or difficulties, but the Lord is able to deliver us. We need to trust Him.