The casks aren't metal on the outside, they're a specialized concrete. These casks have been dropped from building, exploded, struck by speeding trains, and burned at incredibly high temp all without losing integrity. Even still, uranium and similar waste products aren't water soluble and being more dense than any surrounding material so they would sink through the sediment and become locked in place. Here's a really old survey on the topic[1]. Here's another study [2]. Basically most of the waste would be short-lived beta and gamma emitters that are no longer dangerous after a few years. The longer lived alpha emitters aren't dangerous because alpha particles are easily stopped. You can safely hold a small amount of uranium in you hand.

With respect to ocean life, there is also very little life in these trenches, and what life there is doesn't mix with food chains at higher levels. All of the food comes from biological material sinking, and nothing really goes the other direction.

[1] https://www.iaea.org/sites/default/files/31404684750.pdf [2] https://www.swr.de/-/id=13132940/property=download/nid=23345...