And if you can't? Sea tiles need to be more useful than nothing, but less useful than land tiles, that's how it is. For how much - it's the question of balance fine-tuning. If you have a city 5 tils away from the cost, you could build second city closer to the first one with tile overlap or on the coast with free coastal cities. It's really interesting choice, because if you don't consider growing those cities too much, overlap could be better, but if you want to get maximum out of them, grabbing free sea tiles will give you more. I'm not saying that's currently balanced - I didn't play the game. But most of the other posters here didn't as well. If free tiles have the same potential as those you compete for, they are always better. That's simple.