Range and gas capacity is one thing. Sea conditions and wind direction sometimes makes all the difference in how far you want to go versus how far you can go. Running with the wind behind you is deceptively easy....and then you turn around to head back the other way, into the wind and seas, and progress and comfort can be changed dramatically. Get use to monitoring wind and sea conditions and forecasts (which often can change quickly and conditions can be very different from what the weather folks have predicted.) and plan and change plans accordingly.
Assuming you've got the 84 gallon tank...you could go to the gulf stream and back without spending too much time there. However, I agree with mechanic Bob and using the 1/3 rule. So, again assuming you get 2mpg (most likely going to be lower fighting a chop and wind), you've got about 56 miles on your way out and half of your fishing...then you've got 56 for half your fishing and return trip to the hill...hopefully leaving you with a 1/3 tank (56 miles remaning).
As you can see, there is alot of assuming going on. As others have said just KNOW your fuel tank capacity, fuel consumption rates at a couple different RPM ranges and then of course sea conditions etc...
Just be smart and have fun