Fly fishing from a boat comes with its own unique set of challenges. From dealing with boat positioning, to wind drift, to having your line seemingly getting caught on EVERYTHING around you, there's a number of factors to keep in mind. Fishing from the shore can be a more relaxing, simpler way to go about things, but a boat is like a magic carpet that whisks you away to mystical waters not reachable on foot. In addition to some of the challenges I touched upon, just fighting a fish from a boat can be trickier than when you're simply afoot!
One major issue that can occur when fighting a fish from the reel is when that fish surprises you and quickly runs under the boat. Often times, a big, strong fish may even try to take you under the boat and way out the other side! With such an awkward angle between you and the fish at this point, it may be difficult to impossible to control the rod and turn the fish to get it coming your way. In this instance, it's often best to work your rod around the bow or stern and fight the fish from the other side. While this sounds simple enough, the initial lunge of a big fish way under the boat can hurt your wrist, pull the rod out of your hands, or any number of other things you probably want to avoid. I've personally experienced this when false albacore fishing in Florida, most notably once nearly breaking a 9-weight as the fish ran under the boat and firmly pinned the rod between the transom and the starboard outboard.
To avoid such a precarious scenario, I try to remember to adjust my drag during the fight—especially if the drag is set quite firmly. When cranking on a fish, I tend to leave my pre-determined drag setting alone and fight the fish normally. However, when the fish draws near or at any time I feel it could possibly lunge under the boat, I'll take a second and back off the drag a few clicks. That way, if the fish bolts underneath the hull, it's much easier to control the rod and/or work it around one end of the boat since the tension will be lessened due to the loosened drag. Any additional pressure that's needed during this time can be applied by pinching the line or palming the spool, and the drag can always be re-tightened when the fish is back in the clear.