Can I or the coach shout instructions to the crew during a race?

Cheering from shore is fine. Coaching help from anywhere else is not. Under Rules of Rowing 2-409 (Outside Assistance):

• A crew can't receive outside assistance, coaching or advice during a race. Communicating from shore using the unaided voice is still allowed, so cheering and calling from the bank is fine. Megaphones and other amplification are not.
• Coaches and other team members can't follow a race in a launch without the Chief Referee's explicit permission. The presumed penalty is exclusion of the crew and/or disqualification of the coach.
• Parents and other team members can volunteer at the regatta in any role, but they can't assist their own crew unless a referee asks them to.

Behavior matters too. Under 2-410, any team member who behaves in an unsportsmanlike way can have their participation at the regatta curtailed or ended, and any personally abusive language or conduct witnessed by an official is reported to USRowing.