Sadly, I've done this for a few friends. The only way yo do it legally and right is to have your mother cremated, build a boat, and put her ashes in the boat. My wife works for a funeral home, running their crematorium, and it takes a LOT to reduce a person to ashes. Just placing your mother in a boat and setting it on fire would not generate enough heat, and what would be left would not be pleasant.

Check locally, but most states have no restrictions on the transportation or dispersal of human ashes...that's how its OK for folks to spread ashes in lakes, fields, woods et al. Once you have your mothers ashes, you should be fine putting them in a boat.