They don't make canvas sand bags any more... at least not what they sell at Home Depot. The cheap sand bags are now a polypropelene woven bag. Works great for the purpose most people want a sand bag for. If used in your boat, expect them to leak sand out over time.
In my boat, I used the cheap sand bags except I filled them with scrap lead that I picked up (free!) at the local shooting range. If you do the shooting range trick, try to gather the lead from the pistol range and NOT the shotgun range.