Starter sinks in water

Which simple sourdough bread version did you try?  

How old is the starter?  

How do you feed it and take care of it?  How many hours does it take to peak to maximum volume?  

The more details you can provide, the easier it is to help.

or volume measure to feed the starter?  Volume starters at 200% hydration won't float much if at all.  IF you are using volumes 1 Cup each flour and water to feed it is too thin and you need to do 1 c flour and 1/2 c water.to thicken it up some.  Then it should float once it rises.

The floating starter trick will only work if the starter is relatively firm, probably 70% or less.  Yours is 100%.

The rise/rest timing on the Norwich sourdough will have to be extended if you want your loaf raised. You have either low numbers or slower yeast than that used in the recipe.  Nothing wrong with that you just have to be patient with your dough and give it more time to rise.

If you want to speed up the yeast, park it outside the refrigerator for a few days as you build yeast.  You will need some warmer temperatures 75°F or 23°C and a bigger feeding ratio.  Try 1:5:5  for the next feed, mark and time it to peak.  If it peaks before the 12 hr period, discard at peak (not sooner than 8 hrs) and feed again with the same ratio.  If it has not peaked in 12 hrs, let it rise to peak before discard and feeding.  You can save on flour if you reduce the starter size to about 20g and feed 100g water and 100g flour.  A few days of discarding and feeding should help the yeast in the starter. 

If you are gathering lots of discard:  

Example:  400g discard (200g each water and flour)  plus 400g flour    2% of 600g is 12g each salt and instant yeast. (if using tsp, go with 2 1/2 tsp salt and 3 tsp yeast)     

Make a loaf using the most recently made discards,  weigh it, add equal amounts of fresh wheat flour to it, figure 2% on the flour weight for salt and instant yeast additions  adding enough water (potato water or scalded milk or liquid, even toss in an egg) to make a medium firm loaf.  Let it rest for about 30 minutes and then knead, shape and let rise for an hour or so.  

When it has risen about a third and getting puffy, knock the gas out of it and give it folds or a final shaping.  Park in a greased and dusted bread pan or on parchment paper and cover with a moist (wrung) towel to rise.  When doubled or almost tripled (depends on the flour, lower hydration doughs can usually rise longer) pre-heat the oven and bake (430°F or 220°C.)  I like to use the moist loaf surface to cover with raw seeds and then score the loaf before popping into the oven on a low shelf.