I feel like you're all sandbagging the amazing spectacle that was Kristoff's power ballad. It was perfect. The piano, the sick guitar riffs, effects, the backup vocals, everything.
She didn't know what she was looking for. That would've required her to run all over the damn forest freezing shit to find the appropriate memory water. The river/glacier was a nexus of all the memory water. It's also clear that the memories contained within that glacier are a lot more solid and lengthy than the slivers of time that she pulled out of the forest and the ship. As for Anna being exactly where she needed to be, she really wasn't: Elsa shot that magic message before she died and it found its way to Anna, it didn't head straight for that cave at random. They technically could've followed her if Elsa attached a rope from the ice death canoe to the horse. The primary issue with her getting there was that the water horse hated visitors and made the seas difficult to traverse. They go calm after she tames it.
Anna actually being at the glacier would've been awful, though, because then she wouldn't have been able to get back and destroy the dam. So in that sense she was where she needed to be.