Three Items Found...
 

1) Fireworks launch tube. Plastic, but solid and heavy as heck. A bubble kept it afloat just beneath surface in the Broads, where I could just see a corner of the base. :eek2:

2) Some kind of tow board :confused: red and white on the top.

3) Some large deflated bright green float. It's jammed beneath a dock nearby.
(No photo of the float, but I can reach it if needed). :look:

There was a thread here the other day about a missing inflatable raft. I wonder if it is the same one you found.