Just to add another "data point"
This past fall, I tried some heavy benching just for fun. I seldom do bench since it rips up my shoulders -- the point being that since I don't practice bench much, I'm not all that coordinated at the movement. At this point I'm 41 and about 160#. I've done 305# in the past "clean" and 315# w/ some cheating (e.g. arched back), these PBs being when I was in my 30's.
I didn't have high hopes, so I started w/ 250# -- easy, then 275# -- pretty easy, then 295# -- still not too hard, then 305# -- did it w/ some strength to spare so I figured what the heck and went for 315#. That was a struggle, but I did it -- and that was my 5th lift, so I probably could do more under ideal conditions and w/ some practice I could probably add a little more due to improved coordination -- so 325# probably isn't out of question.
Not sure where the improvement came from, but it sure surprised me. I would like to attribute the improvement to apnea sports. I've done weight training on and off for over 20 years and been at various levels of fitness over that tiome (sometimes very high levels of fitness), but I've never had an "out of the blue" improvement like that. The only change has been the freediving and casual apnea practice to give me ok bottom times.
Also, as far as the disbenefits of fast-twitch muscle are concerned. Although they presumably raise basal metabolism, I think that they might not be all that detrimental in apnea sports, particularly with respect to dynamic disciplines. These muscles have a larger cross section, so blood does not flow into them as easily (i.e. they are easy to starve temporarily) and they are optimized to operate in the two anaerobic modes (you'd be surprised how far you can go in the water w/ a little desperate thrashing at the end of a dynamic

). Also, the increased body mass provides additional plasma. and if you can shunt it away from the muscles, then it is useful in apnea events. Contrast this with distance athletes, whose musculature becomes optimized for aerobic performance, developing minimal cross section and optimized characteristics that allow for O2 to be absorbed for use in aerobic glycolosis. Also, their body/brain mass ration is typically lower than bulky guys -- normally a good thing :duh, but probably detrimental when you want to shunt extra blood to the brain.
Bottom line -- weight training might not be good for world-class apneists, but I don't think that it will keep you from having fun in the water and getting respectable times, particularly in dynamics. Also, if Don's experience is any indicator (and based on my experiemnce, I think it is), then the freediving may actually help w/ the weight training.