Welcome to Civ V!
I recommend getting the gold upgrade mentioned already. G&K fixes a lot of the problems in vanilla, and does not introduce too many new ones (impossible coups, and over emphasis on Great War Bombers being the main ones).
The hardest thing for me to adjust to was the 1 unit/tile thing in Civ V, but I like it much much better than the Stack o' Death (now you get to see the ol' Carpet o' Death...).
One civ is as good as any other. Even on a water map you should be able to learn to beat the game on lower difficulties. Just select random leader and have at it.
On a side note, don't bother with Lizzy and her big boats in Vanilla. They can not heal out of your territory, so they just get mashed by a city when they bombard. Naval war is hugely improved in G&K (AI is still bad but better than vanilla), for one thing, all units can fight back and you don't get a mech infantry run over by trireme...