Underwater pictures for swim lessons

I’m building a social media presence for my swim lesson business so obviously, a big part of that will be pictures and videos. I’m not super well-versed with GoPro’s but I have an older version that will work for what I need. I’ve seen some instructors have pictures where they are working with a kid and it appears that the camera is sitting underwater and just angled towards them. I’m trying to think about how I could weigh a camera down there without it moving around. I’ve tried looking for different accessories but it appears that most of the water accessories are there to help the camera float, not sink. Any suggestions? I would love to be able to just put the camera down there and not let it get in the way of my teaching.