Someone in my crew can't make it. Can we substitute a rower?

Yes, within limits (Rules of Rowing 5-206 and 5-207):

• After entries close, a crew other than a single can replace up to half its rowers plus the coxswain. The regatta can refuse substitutions not submitted in writing at least one hour before the crew's first race in that event.
• After the crew's first race in an event, the lineup is locked for the rest of that event, except for sudden illness or injury.
• Serious, sudden illness or injury: with the organizing committee's approval, that rower can be replaced at any time before the event finishes, and it doesn't count toward the half-crew limit. A health professional's certificate is normally required.
• Singles (1x): substitution only for serious, sudden illness, and only before the first race of the event.
• The substitute must be eligible in their own right. In a Masters event, they must be a Master, and the new lineup's average age must still be at least the lower limit of the age category entered. A crew can race in a younger category, never an older one.
• A composite crew stays composite even after substitutions.