Re: How to scrape barnacles off my boat while still in the water?
Well, if you've got a barnacle problem sounds like your anti fouling paint is a goner anyway - wouldn't worry all that much about removing something that isn't working.
But - if you wish to continue just use a 6" flexible metal scraper and have at it. Keep it level with the bottom to prevent any gouging.
You'll also want to drape a rope under the bottom and move it as you go. Tie it off anywhere you can. Cleat, stanchion rail, the rail itself - somewhere.
This will give you something to hang on to. Remember the ole' law "to every action there is an equal and opposite reaction? Applies very well in this case.
Gloves are nice as well. Also tie the scraper onto your BC with some small nylon line. If you drop it, well - you know what I mean.
Obviously since you own SCUBA gear you know the necessity of having someone else there. Besides - makes it easier to move the grab line when the time comes.
