How should I feed my cockatiel and what about their water bowls?

Thanks for all of your questions! You do not need to provide a cockatiel with grit - as a parrot, this material is not necessary for them to digest their food.
For produce, I would recommend focusing on vegetables that are high in vitamin A and calcium - this is typically the brightly colored vegetables (such as bell peppers) and dark leafy greens. For fruits, ideally you would focus on tropical fruits, as they have more fiber and less sugar than our temperate fruits. These would include fruits such as mango and papaya.
For feeding, I would recommend feeding 60-70% pellets, 20% vegetables, 5% fruit, and 5% seeds. An average cockatiel can be offered about 1/4 of food per day - some birds spill more than others so this would just be a place to start and you can then increase or decrease as needed. We want them to be eating portions of each food item, so don't offer too much food, or they will just pick out what they like (typically the seeds and fruit). You can offer multiple bowls of water - often cockatiels will bathe in one and drink from the other. Tap water is acceptable, as long as it is safe for you to be drinking as well. You don't need to offer insects, as the diet that I outlined above is nutritionally complete.
I hope that answers all of your questions, and thanks for using Petco Pet Education Center, formerly Petcoach.