Is the outdrive painted now or unpainted? You definitely DO NOT want to use most regular bottom paints. You will still get build up with an aluminum anti-fouling paint, but some diligence will significantly minimize it. I find I have to take a mask and snorkle and green scrubbie once a week and go over the trim tabs and outdrive. I run the boat every Saturday and Sunday. Even after a good run, if I dive on it, there is still some slime and growth. the anti-fouling paint takes care of most of it, regular running and weekly diving takes care of the rest. If not, it will slowly keep growing. I make sure to pay particularlly close attention to the pickups and make sure those are completely clear.
I split my 15 year old bravo three last year and found approx. 75-80% restriction due to barnacle buildup on the INSIDE of the channel where the raw water is drawn in. I'll be splitting my drive every year from now on as well.