Thank you for your response. I am using INON z330 strobes, fibre optic cable and a Nauticam TTL Converter for Nikon DSLR Cameras. The flash trigger batteries are pretty new. I installed them less than a month ago and they are still showing as good [green light] on the converter when I fire the flash. As I said, this happens intermittently - underwater and above water and seems to go away without any patterns. In the example shown it cleared it up in a minute. Other times I haved turned off my camera and had to wait about 10 minutes (after testing sporadically during this time).
I'm not sure if this is a strobe / camera sync issue or a camera issue.