Hi -
 As I've understood the wiki's, Reservoir is for manmade water bodies that are dammed, Basin is for manmade water bodies that are mostly excavated holes, Pond is for Natural small water bodies, and Lakes is for Natural large water bodies. If it has a dam (which i don't often map much TBH) I select reservoir.
The individual wikis on these as I'm reading now are inconsistent. But if you go to the Key:water wiki, those are the rules I'm following. Just looks like Pond is a little weirdly defined there.