A variety of different aquatic insects lay their eggs on surfaces near the water surface. Based on your description of the insect that you think you've seen doing it, my best guess is that these are eggs of an insect in the order Odonata (dragonflies and damselflies), or  Megaloptera (e.g. fishflies, alderflies, dobsonflies). They really could be from any number of aquatic organisms though. The best way to find out would be to wait for them to hatch and collect the immature stage that comes out of the egg. Those would be much easier to identify than the eggs themselves.