-- Last Updated: Mar-17-13 7:06 PM EST --
...you have some DIY skills. They're cheap, and pretty easy to make, even for beginners.
http://yostwerks.com/SeaFlea1.html
Edit: I see you did specify plastic. But, most people are not aware that boats like this are even a possibility. A lot of people have built these for kids. They're well designed, and don't break the bank.