I have a Blast. Rescued my Catamaran, but sank doing so. It's all rebuilt now, but I think I'll use it to hook a tether line in the future, rather than pushing a stranded boat.
homemade PVC airboat. been using it a few months now, lots of retrievals and its pretty great. Not sure what it cost to make. It wasn't a whole lot. mostly from amazon. its a 3536 outrunner for $20. a 50A esc with reverse. reverse is important. the steering is a robotic servo from amazon I thin $20. 2s or 3s is fine. a 11x5 prop is fine. its about 24" wide and 24" long I believe.
its a fun project and I'm extremely glad I did it.