Ultimately Sunless Sea is a potentially amazing game if you like rich narrative worlds and don't mind reading a lot . . . but you should either wait a bit after release and/or take long breaks between when you play because it's not where it needs to be yet.
Because of the way the game is being developed. Failbetter Game's previous effort, Fallen London (which this game shares the same universe with) has grown massively over time, and so has Sunless Sea. But right now shortly after release, Sunless Sea is a bit sparse. And it's got a lot of issues that need resolving.
The first big issue is the combat, which is very basic at the moment. It works, and it's exciting and dangerous at first, but once you play for a while it becomes very dull and simple save for a few specific "boss" monsters, and even then it's just simple and dangerous. A lot of new AI and balancing is going to be needed before it gets interesting.
Another big issue is size and speed. Your starter ship is almost unbearably slow and at first the world seems vast, but once you get past the (HUGE) initial difficulty wall, upgrading to better ships and engines with a fully explored map reveals that the world actually isn't all that large. More a lake than a sea. Honestly, the game would be better served if the map size were scaled up 15% and all of the engines moved about 10% faster than they currently do. Then at least you'd feel like you were moving, and the world would feel a appropriately large.
The next (and really the biggest) issue is replayability. The game offers a lot of good story content in it, and this content is fixed. However the world is intentionally very harsh and you're bound to die a lot early on thanks to that big initial difficulty wall. These two facts result in a major problem where the narrative content ends up directly at odds with the death and respawn "legacy" mechanics.
Namely: you're going to end up re-reading (more likely skipping through) lots of stuff you've already read before. Finding out about a character's past or an island's secret is really cool the first time, finding out about it the fifth time? No so much. There is currently no mechanic - and certainly not enough content - in place so that the different lives you start encounter notably different story experiences in the game, which would be the obvious solution to this problem.
However, looking at how Fallen London has grown, and some of the systems already in place in Sunless Sea, I feel it's safe to assume that as time goes on more stories and content are going to continually get added in. There's already a pretty big expansion planned, and been a steady drip of new stories over the last month. They even just added a new alternate method to end your current game, so it's not like these are minor updates.
Essentially, what you're likely to not realize (I certainly didn't) when buying Sunless Sea is that this is more a single player MMO that's half Naval Elite/Sid Meier's Pirates and half Victorian/Lovecraftian Adventure gaming. Knowing that is probably more key to whether or not you should purchase it than anything.
This game is currently strong, but threadbare after you spend some time with it. In a few months? I'm guessing it's going to feel much fuller and robust. In a year? It might actually live up to the immense potential it has.
Take that as you will, and certainly, if you're not the type to trust that a game will reach it's full potential in time or don't like that development model, hold off. But I'd encourage you to check back later when it's gotten a bunch of new content.
For myself, I've found more than enough so far to justify the purchase, and have enjoyed the vast majority of my time with the game. The promise of more to come is just a bonus.