You will want to choose a thickness of plate to resist the water flow conditions, then select an acuator with enough force to deflect the plate.
Your stroke length will be set by how much deflection you need. Your actuator type will be selected based on the required accuracy and budget.
Pizeo would work great since it is the highest accuracy and waterproof(probably only very clean water though). I probably wouldnt mess with shape memory alloys as that is likely to introduce more complexity into a simple design requirement. A gear motor linear actuator would work well too if the accuracy is sufficent and it can be kept out of the water. Radio controlled servo actuators come in a wide range of torques and accuracies would also likely work. Possibly hydraulics if the location is to hazardous for electric actuators.
As far as attaching the actuator; the easiest would be in tension like you show. Piano wire could be used to allow the actuator to be located in a more convenient or dryer location.
The actuator could also be in compression from the end if necessary. You just need to make sure the load is pinned and loaded eccentrically so it deflects the same way every time.