What is the best cruise line that leaves from Seattle and tours Alaska?

Sweet P asked:

I am looking for a cruise to take my son and daughter on, ages 14 and 17. They want to sea Alaska, and I would like to leave from Seattle, because it is in the US. I am wondering what the best cruise line is, that has fun activities such as Kayaking. The trip will be at the beginning of August, so I need to book fast!