Just checking late posts here, I would like to add something about

on sea tiles.
They may seem weak, but there aren't many sources or multipliers to

, then there are to the 4 outputs of it

espionage:,

,

and

). That's because they are stronger then those 4 outputs, exactly because they are converted to those depending on the upper-left sliders.
It's true that Trade Routes are far better sources of

then sea tiles, and they are available to any city (unlike

-resources). But cities have a restricted number of TRs available, which in vanilla BtS is 8 (not sure in DoC though). Also TRs can only be exceptionally increased if your city is coastal (lighthouse) and if they are foreign (Custom House), so the bulk of your TRs isn't really great, only the best ones on coastal cities.
A Core tile city that's a Single Tile isle (maybe a little bigger too) is a great source of

because of these 'useless' water tiles. 12 population working these tiles yield 24

, which in turn can be converted to any of the 4 outputs. After being converted they are further enhanced by % increasing buildings like Library for science or Bank for gold. 1 turn at 0%, this same city (which got a market, a grocer and a bank through whip/hurry) will contribute with 48

/turn. And also with 12 population points to the Core Population Score for stability matters. Not so useless now right?