water cooling is better then air cooled if you have a loop that's capable of handling more watts then an air cooler. in basic then, a water cooling loop will take time to warm up so the GPU will take longer to warm up.
1. is the stick pushed down into the slot, sometimes a stick can be a little too big like a 0,0001 mm but it will be hard to push in so make sure you hear a kind of snap.
second use only the black slots so there on the same channel.