with pumps, barriers and quick diggers, say you are on a continental shelf, the sand turns to pebbles and shale, probably pieces of shells and odors from all the bacteria living down there, boulders vary in depth, it depends where really... your question has potential, but you dont really care anyway, do you?
In the UK you would likely be approached by the police fairly quickly in most locations because it is an offence to mine sand, etc. without authorisation (Orkney excepted if you own the Udal rights - they have a land law different from mainland UK) and you would have to explain yourself.