Jon,
In the picture of the frog sculpture where the halves don't meet, the gap was intentional and planned. That design uses another metal piece to cover over the centerline gap to hide it. If I wanted the gap to close though, I would have had to start with a bigger blank with extra metal at the edge to fill the gap. If I'm unsure of the shape, I start with plenty extra to begin with, and trim it away as soon as I safely can.
Your fish form will have some reverses where the body transitions into the tail section, even if the body is straight head to tail. Much more so if in a swimming pose. I find those shape very manageable and fun when approached as described previously. This sunfish is one-sided, and turned to hide the open back. No significant reverses because of the back flipping tail, but the opposite side would have a big reverse. The fins were fun to make.