schottky diode power rating
If the current is continuous at 2A with no variation then 2A is the RMS current. If it's unidirectional (say 20A pulses at 10% duty cycle for average 2A) then the RMS current would be more like 6.3A. 

As to your second question, the datasheet states the thermal resistance as 7°C/W junction to case. If you have to design for a maximum ambient temperature of (say) 70°C, and the thermal resistance case-to-ambient is 20°C/W (just picking a number out of the air) and you are really willing to allow the junction to go to 150°C under those conditions, then you could dissipate almost 3W. It would be more conservative to limit the junction temperature to a much lower level such as 100°C, which would allow only 1.1W of dissipation. 

Fortunately, your dissipation at 2A is only about 0.84W, so it's not a problem, if the assumptions above on case-to-ambient and maximum ambient temperature requirement are correct. The second number should be provide to you, the first is going to depend on the mechanical design and may require building a prototype or FEA thermal simulation. 

17W is kind of a theoretical maximum if you had some chilled water cooled heatsink directly bonded to the case and holding it at 25°C. Not really related to reality.