Does anyone know how to become a longshoreman?

With most unions, you have to start by applying for an apprentice ship position. The openings occur every year or 2 so contact them for the current process

http://www.ilwu.org/

You did not mention where you are but the left side of the page has links to all the locals with phone numbers

for example: Local 30

http://www.ilwu.org/about/locals/local30.cfm

Call Local 333 on Staten Island.

Join the Union.

Also I say become a boat Capt. Better pay.

you need to go down and apply. They tend to get you started on a part time on call status then you work your way up.