I lived up there for a while and knew some people who worked the fishing industry. Work on the boats is usually by shares, you get a percentage of the take based on experience, job, etc. If it's a bad season, you don't make much; good season, big bucks. Work is frantic and often dangerous. Alaska crab fishing is statistically the most dangerous job in the country.
Other seasonal jobs can pay well, but are hard and mind-numbing, like working in the fish canneries. Just check it all out first, including travel and living expenses. And penalties or bonuses if you leave early or stick it out for the season.