Nautilus is a beautiful looking game with deep sea colors reminiscent of Nexus Ops. It is both a tile laying game, a resource management game, and an exploration game. In the end, however, it is a racing game. The theme is about building an exploratory sub-sea station, placing your guys into each module (think Carcassonne), and launching subs to explore and collect sunken treasures and curiosities.
The object of the game is to have the most victory points, which is roughly the product of the number of modules upon which you've placed your guys times the number of treasure chips you have collected from the board. You also add your leftover cash, but it seems unlikely that anyone is going to have much leftover, or even if they did, that it will swing the score.
Like Princes of Florence, cash is extremely tight. Unlike PoF, however, it is hard to get any back. In a four player game, each player starts with 45 cash, but you ideally need about 60 during the game. The only way to get cash is a) for others to land on the tiles you placed and pay you, but that only gives you 1 cash, once per tile per player, and you are just as often going to have to give it back to another players when you land on theirs; and b) collect one of the nine sunken treasure chips, for which you collect 2 to 4 cash depending on if you have progressed in the treasure collecting track (think Goa).
You have fifteen guys to place on tiles and five progression tracks that go from 0 to 4. You progress on a track for placing a guy on the right tile, and once placed it can't be moved. So you will have to forgo at least some of the track progression. But that doesn't matter much.
For each track, you score either 0, 1, or 3 points, 1 if you've reached space 2 on the track, and 3 if you've reached space 4 first and no one else reached space 5, or space 5 first (think Cities and Knights of Catan). Only one person can get the 3 points on each track. But it's not too hard to get to at least 1 on each track, so the odds are very likely that you will be scoring either 5, 7, or 9 for your track progression multiplier. Once in a while you may have to forgo one of the tracks. Suffice to say, the person with the highest score on the progression tracks is going to win, barring extremely bad luck with his discoveries.
The treasure chips are placed face down on specific locations on the board, so you don't have to figure out where they are. They generally yield either 0, 1, or 2 points with which to multiply against your progression track score), or they yield treasure which provides cash. At the end of the game, you can use remaining cash to bump up half of your treasures one point each. Each person is randomly assigned as to which two out of four basic treasures he will be able to so bump up at the beginning of the game. The net result of this is that you need to keep some cash in reserve for the end of the game, and roughly half of what you collect won't be bumpable, so you would prefer to recover the ones that are better for you.
Two mechanics are added to assist you with this. The first one is that the lowest valued treasures allow you to look at some of the face down treasures on the board, but only if you've progressed in the "look at face down treasures" track. The other is that you can spend one cash before moving any sub to look at the face down treasures within your sonar range, typically the nearest 2, 3, or 4 treasures. After a single round of the game, and having not used the sonar ability even once, I decided that this is a colossal waste of resources, as the precious cash that you use just to look at the nearest treasures is better off being used to place more guys on the modules. The odds are very high that you will get something valuable regardless of what you pick up, and all you will have done is thrown out the money which is one less you have to bump up treasures at the end of the game, anyway. That was my impression, anyway.
There are three methods of annoying other players: a) placing modules far away from their guys, so that they have to take a long time to walk their guys over to them (and in fact, may not be able to), b) taking treasures before they can, and c) progressing in the chart and taking the 3 points for that track before they can. Other than that, it's just a matter of counting action points.
I have to say, I don't ever recall playing a game where I really wished for more explosions and combat. Adding a few undersea mines would boost the value of sonar greatly. And adding some missiles and a sixth track for missile/defense ability would made for a much for tense, interactive, and adventurous game. You sometimes wonder if Euro-designers believe that they can't add this as it would violate some unwritten rule.
In my opinion, the game is very nice and should generally play very quickly. The calculations are light, and like any other game with a score based on a product, a square number is the goal you're aiming for.