Morning ! What a thread !
Ok, ex Plod here ...... I take no offence to that term, and it is a term that Police use to describe police ! Ministry of Defence Police are simply known as MOD PLOD for example.
No 'specific' legislation relating to Samarai Swords, they would like other articles fall under sec 139 Criminal Justice Act or indeed Sec 1 Prevention of Crime Act , if used or carried in a public place . A boat is not a public place, so you would not be committing an offence to have them displayed within the boat.
Some good advice so far from posters; make sure your boat is well secured at all times, and that the swords are themselves well secured , preventing accidents or easy removal.
Suffice to say, when transporting these items to your boat, have them well packaged and out of sight.
Ian