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.
2) Some kind of tow board
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).
