The builders probably don’t feel like losing their home and their business because some idiot got drunk and broke their neck in your pool. It doesn’t matter if you sign a disclaimer—you can’t sign away the ability for a guest or even a trespasser in your yard from suing. And if they sue, their lawyer will go after everyone who ever touched your pool.
If you really, really want a diving board, keep insisting. Most builders will give in rather than lose the sale. Expect the deep end of your pool to be huge, though, in order to make diving safer. Consider this: one person using the diving board is tying up ¾ of your pool, while everyone else is in the shallow end. And what do most people do in a pool? Stand and talk, play basketball or volleyball, and float in a chaise with a drink in the arm. None of that requires a deep end, and some of it is hard to do in a deep end. Consider this also: I think that you are out of your mind to put a diving board on a pool.