If your going to be a on a charter boat, they typcally park the boat right above the fish so there really is no reason to cast out. But if you still insist on using your own gear and need to remove the braid, i would suggest you put it on another reel if you have one. All you have to do is have someone hold that rod, loosen the drag on your spinning reel and tie the braid to another reel(of course make sure you have backing), and with then just reel the line from one reel to another. Once done, just add any mono type line. I would suggest 30-40lb test.
If you would be on charter boat often and would like to use your own gear, i would suggest you get a level wind casting reel. Such as a Penn Defiance or Shimano TR casting reel. Both are relatively cheap. And a casting rod(used for boats 6-7ft max).
Just my 2cents...
I wouldn't bother to spool up with 30 to 40 pound mono on a reel rated to hold 12 to 15 for a one time trip on a party boat. If you were going on regular trips i would consider an appropriate rod and reel maybe, but the party boat staff know what works and they they are not thrilled when somebody comes on with something totally different.
They have the rods and reels which can handle the weight range and do the job. A medium heavy powered Ugly with 40 pound line may be ill suited for the task at hand especially when your not sure of the conditions. A full moon is upon us and that will make for even heavier sinker loads.