To the OP:
I agree with what several have said regarding the big, yellow winder: ditch it. They're big and bulky when compared to other options, and you can only play out line in sections that match the length of a half wrap on the winder (6" or so, possibly double depending on how you secure the line to maintain tension). Slightly 'floaty' rope is only helpful if you have too much of it played out, which you should never do anyways.
Regarding the 100' finger spool with the special yellow, folding winder knob thingie:
Had one. I wound up popping the winder thingie out from the spool, and trashing it. Finger spools are simple to use, IF you understand how to use them properly. It's not as intuitive as some people think. The winder thingie just complicates things.
If I was only ever going to be tying off a flag at a dive site (not towing one) I'd just keep a dedicated finger spool with the flag. In my area, however, there are some dive sites where it's much safer and reasonable to take the flag with you. These are (in my area) shore sites with multilevel profiles. Because of this, we use a reel. Not a high quality reel that's appropriate for navigation or overhead diving, but a relatively inexpensive one like this:
Ratchet Dive Reel, Yellow
We secure a small SS bolt snap to the end of the line, and simply clip to the bottom part of the flag's shaft. Having the small bolt snap also helps in carrying the flag to/from the shore site. Attach the reel to your hip d-ring. Upright the flag. The shaft weight will cause the bolt snap to rest directly against the float, making the flag stay upright and off the ground as you're walking. Helpful if you have to climb over rocks, or carry other equipment to the water.
I'll dive with the reel in my right hand, with my thumb next to the release lever. Super easy to play out a couple of feet at a time as you descend. The fact that the reel self-locks every couple of inches is nice when you're ascending and cranking it back in. I've had no issues with one of these jamming - as long as appropriate tension is maintained. Also, make sure that there's not too much line on the spool. Most of them come over-wound: It helps if you remove about 20' or so of line.
Above all else: practice. Flag bearing can be a fairly time-consuming activity, depending of the site. Learning to maintain appropriate tension on the line while ascending is important. Too much slack, and you're swimming in a large bird's nest of line.
Carl