H-in-a-circle symbol on a passenger ferry boat

[NY]

I'm familiar with a large "H", often inside a circle, painted on the ground
to signify a helicopter landing pad.

But what is the significance of this symbol, about three feet in diameter on
the deck of a passenger ferry boat? I saw one today: three-foot diameter
yellow circle painted on the red-brown deck, with "H" in deck colour inside,
almost filling the circle. Given that this was a small inter-island
passenger ferry in the Orkneys, that the circle was only three feet in
diameter and that it was right next to the side of the boat, it wasn't for
landing a mini-helicopter :-) So what *was* it there for? I'm intrigued.