50 miles "could" get you whee you need to be on the right day...
Your best bet would probably be a very very earliy morning run to the shrimpers when they are dumping..

As far as Galveston, we usually have to get out to the big break which is 100+ miles. I am speaking of YellowFin, I would be you could find some black fin in closer. Anytime you get further than 30 or 40 out in a relatively small boat, you are sure pushing your luck though. I speak from experience, having got caught in a freak August storm in my 21 foot Cobia. Man that was a brutal, long ride back. Did not think I was going to get my boat back. It can be done though, you just have to be ultra conservative.

We caught our bait on that trip to BoomVang on some grass lines about 50 miles out, on the Sabiki rigs. While the rigs are never 100% they are pretty reliable when jigged around bouys, grass lines, etc. You can also beat up on tuna with diamond jigs (at night), topwaters and floating flying fish, and best of all cut bait chunks. I would recommend finding yourself a guide down there that you can hire and then fish with him and learn!