Actually, to collect water samples you don't need a pump or even a servo, especially if it's one sample at a time. All you need is a small container suspended under the drone. Drop that in the water body to sample, it fills by itself, and you're good to go. Maybe add a little mechanism to open and close the container, but even that can be done by the container touching the water (have the container sink, little float to the lid to pull it open).