There is something wrong and it's very unlikely that it's your batts. That starter is a .8 Kw and should easily be started by even a personal water craft battery.
1) Starter ground to engine (it is case grounded make sure it is clean)
2) All connections
3) Circuit path to starter
5) Battery switch
6) Glow plug circuit and glow plugs. (it is not clear whether it is not cranking or starting)
Also post this over on the C-34 forums (LINK)