It sounds like your game is more about the scenery and exploration than about (networked) interaction with lots of entities. And being coop, you don’t have to worry too much about fairness either, so it’s a good starter project for multiplayer.
If exploration means you generate the levels, then keep in mind that you can sync the seeds of generated locations, if the generation is deterministic. Otherwise, syncing generated levels is a pain (due to the amount of data you may generate).
Most networking solutions will do for this. Out of our products, I would recommend Photon Bolt. There are samples on our page and an advanced sample package can be bought in the store, too. While some other solutions will be absolutely free of charge, those don’t come with hosted matchmaking and relay.
Edit: Here’s a commented list of networking solutions .