Go on board an ocean liner or large-sized fishing boat and see if you can find a railing system fabricated from any other material. How much time do you have because this is an enterprise that is likely to take you the rest of your life. The fact is you are never going to find a railing system made, say from wood or plastic on all the ships and boats in the world. But for each ship or boat that you board, you will be touching scale with a steel or metal deck railing system.
Take into account the likely scenarios then. In the case of a railing system built solely from wood, can you only imagine the consequences? How long is such a structure likely to last when you take into account how much regular force is being applied to it? And even if, say, the small-craft owner was nimble and light hardly the case if he is going to be a seasoned ocean-faring captain, but say now what do you think is likely to happen if ever the craft should encounter some or another form of collision at sea?
The collision does not need to come from another craft, that is likely never going to happen approaching craft would have been sighted from a fair distance but from none other than strong and high waves. Apart from the entire craft capsizing if it were to be made solely or mostly from wood, and this has happened, the railing system would have been crushed into thousands of useless splinters.
That is not and never likely to happen with a metal deck railing. And just think how much all other land-based structures and constructions would be benefiting from such materials.