‘Zero head’ turbines can be placed in rivers, or other flowing water, and will respond simply to the flow - producing a small amount of electricity for trickle-charging a 12 volt battery.
For example, the Ampair underwater generator will generate up to 100 watts in fast flowing streams.
It is also possible to purchase turbines that have been designed to be towed behind yachts.
Archimedean Screw turbines are also good for low-head situations, easpecially those with heavy debris.
See the question about converting old mill sites for more advice on 'low head' sites.