You might consider sinking your cinder blocks first, and then attaching the fountain.
Depending on the depth, you could run your attachment through the cinder block, sink it, then repeat the process for the other cinder block. Then you've got them where you want them without having to wrestle with the fountain. Then just attach the fountain.
That's exactly what I was thinking. Maybe I'm overthinking it, but here was my plan: first mark the middle of the pond by running a rope, marked at the halfway point, taut across the pond with an equal amount of excess on both banks. This rope will also come in handy to secure my little raft and fountain as I work. I'll attach fountain to midpoint with a bit of tape, head over to one side to drop the first anchor and line, then attach the anchor line taut at fountain. Then head over to other side and repeat.
