Can I delete a wave or a complete surf session?

Yes you can. On your iPhone open the surf session on the map. Now in the bottom panel you see your session summary. Use the “…” icon to delete the session.

To delete a wave, select the concerning wave on the map. You can also swipe this bottom panel to “page” through the waves you surfed during that session. To delete a wave, select the concerning wave and use the “…” icon.