Suction cups don't work - if they did the bottom wouldn't need
scraping.
The suggestion of using a hookah unit is good - I've spent as long as 5
hours underwater scraping our bottom without surfacing. The freedom of
diving without the tank(s) is also an advantage.

One trick is the kind of scraper you use. It has to be HEAVY. I use a
cold chisel - it has a steel handle about 1" in diameter (hexagon or
octagon?) by 15" long and a blade about 4" wide and 1/4" thick. The
advantage of this scraper is it has its own weight, its own inertia.
If you use a conventional paint scraper you are relying on the mass in
your arm to push the scraper through the barnacles and that continual
shock will make your arm very sore very rapidly. With the heavy cold
chisel it absorbs the shocks as it bumps the barnacles off.

Ann Marie