look at alkathene water pipes, blue or black. They are relatively cheap, various diameters, and can have an 8mm rebar inside for rigidity. Also, various fittings are availabloe for cross pieces or diagonals.
If you are anywhere near Exmoor I have some old lengths lying around.
Thanks for the replies (boat is Essex based) but I've seen some others use something like small bore plumbing pipes; I guess a visit to B & Q for some inspiration is required!
A while back I stuck some bamboos from the garden centre into an 1 1/4 inch plastic waste pipe to stiffen it up as a ridgepole for a boat winter cover. It kind of worked for a few years.