what i did follow me on this one
i got a white backup light for a truck thats completly sealed,
i made a u bracket and screwed the light to the bracket, i made the bracket out of scrap metal band the i had
next i mounted the light and bracket to aflat piece of wood with 1 screw so it can be turned and attached wires with alagator clips and just attache the clips to a battery which i take onboard so i dont drain my battery and it lights up the whole deck and comes of at night when i go home
it sounds complicated but its very easy
oh yeah i got the light from a trucking comp that i know very cheap cost wise
now if your looking for a spotlight to mount to your rail i cant help you because i want one too
i seen them on boats but cant find them anywhere
deck light no problem
spot light a problem,i would just make one myself