I have heard this theory about them being too heavy, something to do with subcutaneous fat levels?
has to do with having more muscle and less bf.
when i was younger, i was in the water daily.. now, fast forward 20+yrs and after 19yrs of training, having not been in the ocean for nearly 10yrs (and having a good deal more LBM than the last time i was in the water), i go to swim and holy fuck... i can barely tread water without sinking. fat floats, muscle doesn't.... was quite the unpleasant and frightening surprise.