Here "baptism" refers to what Jesus must suffer. Just as water covers a person during baptism, suffering will overwhelm Jesus. Alternate translation: "I must go through a baptism of terrible suffering" or "I must be overwhelmed by suffering as a person being baptized is covered by water" (See: [[rc://en/ta/man/translate/figs-metaphor]] and [[rc://en/ta/man/translate/figs-activepassive]])
This exclamation emphasizes how distressed he was. Alternate translation: "I am terribly distressed and will be so until I complete this baptism of suffering" (See: [[rc://en/ta/man/translate/figs-exclamations]])