: Given that paddle blades need to be thin and resistant to
: splitting, is there any reason - apart from weight - why marine
: plywood is not used or rarely used? I would have thought it
: would save a lot of work and produce a more robust paddle.

I can think of a couple of reasons:

1- You can only bend it in one plane and unless you're using thin ply, you may not be able to bend it much.
2- You can't sand the surface to shape, since you'd cut through the plies and it would look terrible.

There's no reason that it couldn't be used for very simple blade designs as long you seal the edges well, but you can't do much with it.