I presume you are looking for something like this:
as would be found on a caravan... (you can also get amber lights but I think they are intended for LGV use?)
The problem is these aren't waterproof so will get knackered when you launch and recover. Boat trailers are not required to have side lights (wheras other large trailers are) for this very reason. Keeping salt water out of the lights / cables / connectors etc will be tricky.
I think you either will need to rig up some clever removable ones (sounds like a pain) or settle for not having any like eveyone else. Perhaps some extra reflectors would improve visibility?