There is a maximum of 10 skill levels. The skills are calculated 100*level. This means that you have to do 1000 activities in one area to get level 10.
Depending on the level, there is a multiplier in the earnings. This is 0.25 + level. This means that at level 10 you have a multiplier of 2.
The skill level is available for the following activities: Fishing, Sandbags, Salvage, Harvest, Crystal, Ore and Crafting.