Dry bags sound pretty straight forward but they can be a minefield.
There are very lightweight bags that have sewn and taped/glued seems. These are great for cycling and walking but tend to get damaged too easily to survive on a SUP deck.
I prefer heavier bags with frequency welded seams.
For snacks keys and phones: 3 to 6 litres is a good size.
For spare clothing and cold weather gear you should start at: 10 litres.
I can get a pair of heavy wetsuit boots or Cag’s, jumpers, gloves, hats and a buff in one.
For camping you probably want to vary between: 10l and 30l. Don’t go too large as it is better to have a few smaller bags, but not too many. Larger bags are also harder to keep waterproof.