Depends on a number of factors. What voltage will these lights run on? How many lights? What is the wattage of the bulbs you will be using in these lights? If these are line voltage (120volt) are you going to be needing outlets at some or all of these lights?
it will be 110V and what i'll probably do is put a two-outlet receptacle near the pond where i can plug lights and whatever else i might want to plug in. i don't plan on using anything with too much wattage. right now i'm using three 50 foot extension cords plugged together and haven't had any problems.
Conduit or direct burial? 200' is a fair distance and could be a factor in determining your wire size. Since you are using extension cords and they are not tripping breakers that is a good thing.
I would run #10 wire in PVC conduit to a weatherproof 4x4x"deep" box outside. Use a 20 amp single pole recept. Then you can use #12 for additional lights ect.....
The #10 wire will give you some room if you ever need it. And yes, it needs GFCI protection.