A lot of it is to do with personality - what could be a minor obstacle for one child can be a major one for a more cautious child with which they need a bit of help to remedy (ie the actual degree of dyspraxia, or hypotonia, or whatever, needn't be that great, just enough to put off a cautious child from trying). Fwiw, my ds1 has a super-cautious personality and was incredibly fearful for years about putting his face in the water, then about jumping in, then generally about sinking. He is now (age 7, going on 8) pretty confident in the water, despite his personality, being extremely hypermobile and having slightly low muscle tone. He can swim a length or two of the pool on front and back (can swim further, although his low tone and hypermobility mean he tires much more quickly than most children). Having individual swimming lessons with someone he trusted and felt safe with made a HUGE difference. Group lessons were a complete waste of time and money for him.
I spent a LOT of time with ds1 when he was little, helping him climb things, pedal, ride scooters, etc, (otherwise translated as not letting him give up trying) because there was an obvious physical need to keep him active and help him build up the strength he needed to take part in that sort of exercise (he also needed help learning to crawl, roll over, get to sitting, cruise, walk and pull to stand). He's also had quite a bit of physiotherapy. The difference it has made to his physical confidence has been huge, though - he will test his physical abilities much more these days and really enjoys running about, rather than automatically favouring opting out. He can ride a bike, he loves his 2-wheel scooter, he can swim and what's more, takes great pride in the fact he can do these things, because they did not come easily (only came, tbh, because nagging mother wouldn't let him give up). The more he achieves physically, the more he is willing to try and the more confident he is, physically and socially.
ps I wouldn't be surprised if small, thin children find it harder to float in the water! Ds1 is actually very tall for his age (albeit also very thin), and his length seems to help with the swimming - he can go surprisingly fast. Ds2 is more petite and seems generally less floaty unless he has a large lungful of air!