Every year a couple of my buddies and i do a 3 day trip for tuna, mahi-mahi, marlin etc out of hyannis mass.
We leave the dock at 8 AM the Tuesday morning after labor day, head out to the offshore canyons and return on Thursday afternoon.
Last year we hammered yellowfin tuna... including a 3 hour stretch in the middle of the night when the fish were schooled right under the boat and it was like takin candy from a baby.
We ended up catching 18 tuna between two of us.
Boat is the Helen-H .... www.helen-h.com ... takes out 30+- guys and there is still room if anyone has any interest.
While I cannot attend, I can say that I used to live on Cape Cod and went on many trips like this one. In fact, my bother married into a family who owns a smaller deep-sea fishing vessel he captains for much smaller groups and he is out of Wellfleet.
These trips are a lot of fun and can be a lot of hard work as well, nevertheless they are a great experience and you won't be disappointed. If you have the time to spare you should contact John and go on this trip. You won't regret it.