Personally I think swimming diaper is better than swimming pants, you can just throw it away after using.
All the swimming pool has baby size life jackets, then you even don't need buy those 连身的游泳衣,前胸和后背里面都有类似泡沫版的东西,有两种size,分男女款式 (they are too big for baby under 1, I found)
My daughter has a 连身的游泳衣, but she starts using it after she was 3.
BTW, never seen the one you mentioned which can be 套在婴儿脖子上.