A semantic conundrum

My run to work now passes through Linear park by the side of the Torrens river. Today I noticed a sign that got me really thinking.
The sign said:
"Carpark here. No horse floats beyond this point"

My problem was: is the word "floats" a noun or a verb?

• If verb: do people in Australia (still) throw horses in rivers? If so, why is it important that the horses no longer float beyond a certain point in the river? Huh?
• If noun: What exactly is a horse float? I imagined something like a hydrobike but with a horse head on top (made of plastic, of course, this is not Italy). But if so, why would it matter that only horse floats were not allowed beyond that point? Would a normal hydrobike be allowed? What's with the discrimination against horse-headed hyrdrobikes?!