This is viewed as a royal psalm, a psalm about the king.
David composed it after deliverance from his foes according to the heading. 
Look at how he describes his relationship with God in vs. 1-2.

18:1-3 He loves the LORD

18:4-6 The crisis from which the LORD delivered him

18:7-15 The LORD's dramatic intervention to rescue His servant and judge His foes

18:16-19 The LORD drew David out of the waters