Our boat is equipped with a flex coupling made by Federal, as you see here:
As is often the case with this kind of job, there is very little room available to work, and there was no chance of fitting a conventional jaw-type puller. So what I used was a plate puller designed for removing flywheels from OMC outboards. The particular one I have was my Dad's that he used for servicing Gale outboards at his dealership back around 1960. The design has been unchanged all these years, and the exact same puller is still readily available today, it is extremely robust, here's a picture of mine:
It bolted right up to the coupler using some 3-1/2" 3/8" bolts. A few turns of the big center bolt and the unit was clearly moving.
There wasn't enough thread to pull it all the way off in a single setup, so it had to be removed a couple of times to add spacers inside, in this case a socket and a few washers, but it worked perfectly well. I honestly think you could not design a tool specifically for this job that would work any better.
This can be an awful job, but in this case, the right tool made it easy.
Tom