Why not consider making one out of foam, either block or built up using pink or blue insulaton board? Its easy to sculpt & conture into the shape you want & will certainly never sink! Just be sure you know what foam you are using before you try to fiberglass it - Styrofoam or polystyrene foam doesn't take well to polyester resin but can be used by first applying a barrier coating. Here's a good link for tips on using the medium.

[link=http://www.sculpture.net/community/archive/index.php/t-2182.html]Hotwire Foam Tips[/link]

I'm playing with it now between kits to make a simple Chinese river barge for the pond.

Chuck