CKD stage 3b here. At stages 3, 3a, and 3b you shouldn’t worry too much about food restrictions. Cut down on salt added after a meal is cooked; avoid processed food and ready meals. Your blood test results should be on your NHS app or, if you don’t have that, your surgery should be able to provide you with a printed copy. The important number to look for on your results is your eFGR, your estimated gromular flow rate, which will tell you how well your kidneys are working.
You don’t necessarily have to drink just plain water. My GP said all fluids count, whether it’s squash, tea, coffee or soup. They all count. I drink a total of 2 - 2.5 litres per day, a mixture of all the above. He said to make my drinks pleasant, not boring - unless, of course, water is your preference. You should, however, avoid fizzy drinks, especially cola.
I have the occasional banana, still eat the odd tomato, and every now and then a jacket potato. All foods that contain potassium, but you can keep an eye on your potassium levels on your blood test results. You need some potassium for your heart’s sake.
Your blood test results not only tell you the levels of the various things in your blood, but they also give the acceptable ranges of them, so you can see if you are higher, lower, or within those ranges.
You may find this chart useful.