Staale
You are right in saying that Guam is part of the US. You will need a "C1D visa" (crewmans visa) to enter if you are getting onto a boat but this will be arranged when you have a boat to get onto ie a job. To go there otherwise you may or may not need a visa depending if your belong to a country that is part of the 'Visa waiver programme'. The company that you are going to work for will send you a letter that states that you need this visa for tuna boat work through Guam and will name the boat you will be getting onto. You then take that letter to your nearest US consulate and apply for a "C1D visa". You will need this letter when you enter Guam as well. All this will be explained to you by the company you have a job with.
Good luck
If you have any other questions PM me if you want.
Grogan