Mike -- here's what we have tried the last few years...of course we have probably lost brain capacity due to lead poisoning, but what the hey, its for wakeboarding!
Sand -- too big and too messy even in the tubes. We keep a bit just for easy shifting and balancing the wake.
Lead shot -- from Walmart I think? This is great and comes in canvas bags. Note to self...canvas rots with water & sun exposure and lead shot is a b*$%# to clean up...Don't drop them overboard onto the driveway when you are vacuuming the boat....and don't drop them IN the boat b/c they will burst and spread lead shot all over the carpet.
We tried wraping the bags in duct tape. This works good although if you compact them they loose their flexibility and are like rocks.
Lead bricks - get an old charcoal grill some bags of lead shot and a few old metal breadpans. Pour the lead shot into the bread pan - heat to liquify and cool. These make awesome little bricks...I'm not sure how safe this is to do...
I'm a chronic complainer about the lead residue in the boat - it can make a greasy, grey mess. So the latest and best method we have so far is to make the "lead loaf" as mentioned above, and then coat it with spray on rubberizer from Home Depot. I'm not sure that it protects us all from lead poisoning but the boat stays a bit cleaner. I guess you could use any type of heavy duty baking pan - a cake pan would give you more of a lead plate shape. At a clinic I went to they had a plate and covered it in carpet - it looked great.
Good luck - and you will curse like no tomorrow the first time you stub a cold, wet toe on lead. After that you will look where you are going and it won't happen twice!