Yes, nothing happens at slow speeds with (almost) any generator.
See that idiotic "kv" rating on the spec sheet? I HATE the term. HATE IT!!!! "Kv" means "killivolts"!!!! Some jerk decided to use this instead of RPM/Volt, which is what the spec really means. So, for your motor's 850 RPM/Volt rating, at 9.6V, it would be running around 8,160 RPM.
I don't know what the pitch of your prop or your flow rate is, but you aren't going to notice much spinning this or almost any other motor by hand without gearing. Get it spinning a few hundred RPM and THEN short the windings.