s.bernbaum
Mostly lurking
Lymond answered all but I have the impression that you are just beginning with CIV and a little clarification may be in order.
The strength that you see for the unit is the base strength. If you point to the unit you will see a box appear to the left of the screen showing all of the bonuses that the unit has. These will be added to the unit strength when it is in combat. (Actually some of them decrease the strength of the opponent but it is simpler to just figure that they increase your unit's strength.)
Yes. You need to use the work boat to get the resource benefit.
You almost never decrease unhappiness. What you do is balance off unhappiness with happiness resources. The goal is to have more happiness than unhappiness, so the net result is a happy city. However, if you use slavery to build something, the decrease in population will decrease the "it's too crowded" unhappiness by the number of population points used, although using slavery adds a slavery unhappiness point. So, in that case you usually want to use more than one population point for the slavery build to end up with less unhappiness (also consider that using the slavery build will make your city smaller, until it grows back).
Why do combat unit's strength ratings never go up when you give them a +20% strength experience upgrade, even when their starting strength is 5 or more?
The strength that you see for the unit is the base strength. If you point to the unit you will see a box appear to the left of the screen showing all of the bonuses that the unit has. These will be added to the unit strength when it is in combat. (Actually some of them decrease the strength of the opponent but it is simpler to just figure that they increase your unit's strength.)
Also do you need a work boat to work each sea tile to get any resource benefit from them? Or do you get the bonuses whether or not there's a work boat in the tile?
Yes. You need to use the work boat to get the resource benefit.
And what can you do to reduce unhappiness caused by citizens of a town saying "it's too crowded"? Does building cottages help? If not, what can you do to reduce "it's too crowded" unhappiness? Or is that just a placeholder text to cover all kinds of other causes of unhappiness?
You almost never decrease unhappiness. What you do is balance off unhappiness with happiness resources. The goal is to have more happiness than unhappiness, so the net result is a happy city. However, if you use slavery to build something, the decrease in population will decrease the "it's too crowded" unhappiness by the number of population points used, although using slavery adds a slavery unhappiness point. So, in that case you usually want to use more than one population point for the slavery build to end up with less unhappiness (also consider that using the slavery build will make your city smaller, until it grows back).