cedar shingles do not last a life time. they do get brittle and need replacing. Hail or a base ball will wreck a shingle real fast. Sometimes they are cut too thin and the sun over time will cause them to cup and then break under the pressure.
I feel that the first task is to replace the faulty shingles, which should be an on going maintenance project.
There is a super stain/paint for cedar shingles and it comes in all colors you want. Sherwin William's sells it and its called Woodscapes. Its great for this and can come in the white wash effect. However it has to be sprayed on, or its easier by far.
Being by salt water this will put undue stress on the shingles and cause them to prematurely age. I would consider the staining about every 7 years.