I built a hack to use my smartphone under water https://forum.inaturalist.org/t/life-hacks-for-naturalists/4636/95 from pondside/ dockside/ or boatside. The video footage turned out quite well.
One could make something larger by gluing plexi to the bottom of a bucket. Check it out for leaks first well before you place your dslr in it. These hacks are intended for those who can’t afford to get the latest and greatest equipment and probably cost less than $50 and time.