What are the downsides of dumping trash in ocean?

So dumping garbage has been messing with beaches and ocean life for a long time, but what if the you sink the garbage in absurdly deep part of ocean? For example, if you got a cruise ship sized boat that is designed to keep garbage from escaping and when it is full you take it to someplace very deep and sink it. Is it because it’d cost too much or would it still have too many negative impacts to the ocean? I mean if it’s something you plan to sink you could cut costs on tons of stuff. Just too much trash? Same with radioactive waste if put inside something airtight that would keep the waste from touching water.