I have a small pond that i am going to put Koi fish in what plants should i get?

I know i am going to get some lily pads and some water lily's but besides this i have no clue. i no my pond needs lots of shade but my pond is in the wide open so i am counting on plants for the shade. also Should i have some kind of bottom for my pond? there is some dirt on the bottom but u can see the liner mostly. i was thinking like pebbles what do u think? Any suggestions or ideas for my pond please include. right now i have a 300gph pump fountain. it also filters the water. Thanks