You start with an engine, a dive shaft, and a propeller. Simple and beautiful. ...and then it starts to get complicated.
You want to be able to go fast when you need to out run a storm or meet a deadline. You want to be powerful when loaded down in heavy seas off a lee shore. You don't want to waste fuel. You want to optimize free wind and solar power. You want to be able to fix it yourself. You want a backup plan if fixing is not going to happen quickly.