- You could have some manacles in the bilges where miscreants get shackled if they cause too much trouble (a hook for a rescue?).
- A sign on a porthole saying something like 'vomit here'. (a bit of local technicolour)
- Rats and cats.
- If you want to get a bit silly, one of the figureheads rigged up in the middle of the bar like one of those bucking broncos, with ropes attached to it so people can pull it around to topple the rider.