Why would you want to attach a fishing line to a float to the surface? Seems like it is better to keep the helicopter at the surface, so just attach this device to the top of your boom and it will float the whole unit. 2.2 pounds of flotation is plenty of flotation to float your average multi rotor with camera. The total weight of your helicopter and camera is not relevant; it is the density of the total unit relative to either salt or freshwater that matters here. A couple pounds of positive flotation will keep a 200 pound human afloat, but it obviously won't float two hundred pounds of lead. Since these units rely on batteries it seems like two may be wise for redundancy.
Good point Shane but i'm not entirely sure how dense Carbon Fibre is but I can't imagine it possesses much buoyancy.(what do I know though) How about mounting one of the inflatable key rings in the end of each boom, that would give you 20lbs of buoyancy. Second thoughts the props may pop the balloons....hmmmmm NIck
Not sure if this helps but would low density floatation foam poured into the booms be a possibility? Take a look at http://www.aeromarineproducts.com/boat-foam.htm It doesn't give floatation specs but it might be work a look at.
It would help. But wouldn't keep it afloat. If sealing up 100% of the air inside won't keep it up, then just having a partial amount sealed in the booms wouldn't (which is pretty much all foam is doing).