Concerning sea resources: like land resources, I think they should be accessible by connecting to them or building a fishery/platform/mining colony or whatever to it to utilize it. For a long time, you could only get to nearby coastal and sea tiles, but later on you could get to deep water resources, not just depending on a city radius. I too love the idea of many different kinds of and degree of resources and luxuries which are not dependent on a city being so nearby to it. Some resources should have common values and uses and fewer ones would have rare or luxury values, many of these shifting in later eras as well.
In Civ Call to Power I liked how they had squids, crabs, and whales as sea resources as well as being able to harness the production of underwater rifts and seamounts (volcanoes). I was thinking that offshore resources should be available if you want to bother to send a ship out there and invest in building something (fishery, oil platform, etc.) and then youd have to protect it from being pillaged during wartime too probably.
You could even fish international waters if no one else has claimed a pod of whales way off the coast. Might be hard to hold on to them during times of war though.
Instead of needing just one oil to have all the oil youd ever need, resources would be more plentiful and spread around in the appropriate terrains, but represent finite quantities each. If you are going to build a huge tank army, you have to have some number of oil sources to add up to enough to fuel them perhaps. The more excess oil you had to trade, the more money you could bring in and buy things including military units from other nations, like rich Kuwait which has nothing else much going for it and is in desert terrain, but is fabulously wealthy. It remedies the all or nothing approach to resources and makes it useful to try to get more and more of them, rather than just one or two of each. it might take a certain amount of the same luxury item to satisfy all your cities if you have many large cities too....more than just one source...maybe two or three or more, depending on how proliferous they might be as well.