I'm about to create a new adventure for my players, the begging of the adventure is that they came in this new word, unknown, but very similar to eberron / faerun 

They spawn in the middle of the sea, after a vivid dream about the multiverse and a mage conjuring theme in this new word

The will be rescue by a fishermans ship, after a few dialogues they will drag into an adventure in the sea, secret island ecc ecc

My doubt is : when they are conjured in this new word they maybe want to explore, to know why, who is the mage.... is correct to stuck them in a ship, in a journey? How bad things could be? The take the ship, kill everybody and ignore the sea quest? Is that a bad opening? 

If they are 1st level, then there should be enough crew on the fisherman's ship to prevent the idea of taking it over a thing.

If they do take it over, then authorities are going to be looking for a missing ship.

Being rescued by a ship is a good thing, it allows you to get them to a specific port/town early on.

Will the ship owner ask for some payment? Or ask for help with some other problem they are having?

That's for sure "you are on my boat, you have to work somehow" that's the start for the adventure