Try anchoring a cable over head set a pipe or pole in concrete  brace your poles with 4 guide line  should work just fine. tell me if I win.

It be cool to see some pictures of what your working with

At least you are using steel so you could blow it out with water or Air

DEP would be all over you if you tried to clear a plug over top of the water because its a "Hazardous material" LOL. IDK maybe your local laws are different. wet concrete spilled in the water is a big no-no around here

Feasible to get several little cheap boats...maybe one under every 2nd or 3rd clamp?  Should give you a bridge of sorts....70 meters with a good sloppy yard of grout should prime no problem. 

Pumpnfinisch: thats were the price of the project is set on I think

I want to see pics of the waterside

What about some floating pontoons