This foam football costs @ $6 http://www.walmart.c...otball/16504574 Use a tapestry needed to "sew" a length of monofilament to the bottom (width) and attach a few lead sinkers. Then do the same to the top, attaching a float, to a fishing pole.
With a little bit of effort, you can determine "zero bouyancy", and lower it to a depth between 1 to 1-1/2 ft below the surface of a pool or lake, so you can safely practice blowgun fishing. Once you become proficient with the practice target (check your local fish and game laws), you're ready to "bring home the bacon" (uhhh, fish).
*NOTE - In a real life fishing situation, unless the fish is within 6 to 8 ft from your barrel muzzle, the effective dart penetration below that depth, is negligible. Your dart willl be annoying the fish, not catching it. Surface feeders, like crappies are the exception. I've seen them taken at about 25 + ft. distance with a bamboo skewer dart.