Like Peasant says check the beam pattern on a wall, or just look at the lenses. There will be an obvious prism on one side to kick up the light to either right or left. It should be on the same side as the steering wheel IF the lights are original - haven't been swapped for RHD units.
If you have RHD lights (prism on the LEFT as you look at it from the FRONT)then get some deflectors or some insulation tape and mask that bit off.
If you have LHD lights (prism on the RIGHT as you look at it from the FRONT) then no deflectors needed, but it
should already have some on for driving over here- in which case take them off when you get on the boat. You might need a sharp knife to scrape them if they've been on for a while and generally they go brittle and fall off in lots of little bits so you won't be able to reuse them when you get home
There's a light hearted explanation on here
http://www.aer-net.co.uk/sdc/headlamp-conversion.htm
Hope that makes sense
Andi