Rule 1: It's not about you.
Your support and encouragement are incredibly valuable to your rower. However, as life at sea is incredibly tough, and sending messages 10 times more onerous, and 50 times less immediate than it is on land, craft your messages (content and frequency) based on what THEY would enjoy receiving not what YOU want to send.
Everything else derives from this rule!
Here are some detailed Dos and Donts for how and what to communicate whilst your rower is at sea. Be careful, though, because you may find that you develop OARS (Obsessive Atlantic Rowing Supporting) or the related condition SPOT (Serious Pacific Ocean Tracking) Syndrome!