Yeah, I'd, as already suggested take the nanny Catch road/track. I used the Flat Fell route last time, nothing interesting up there, went wrong near the top too.
The path directly out of St. Bees towards Stanley Pond saves a lot of time.
As for water bottles, my sack is an Osprey with front as well as top access so is easy using one hand to pull out and and back in, on the move