Treasures are small and rare enough so you can never rule them out completely, but if there were significant amounts of rocks there, we would know because Dogge

Treasures are small and rare enough so you can never rule them out completely, but if there were significant amounts of rocks there, we would know because Doggerland has been fairly thoroughly “ploughed” by fishing trawlers and mapped for research (https://en.wikipedia.org/wiki/Doggerland#Discovery_and_investigation_by_archaeologists https://en.wikipedia.org/wiki/Doggerland#Discovery_and_inves...) The geology of the North Sea also indicates “sand” more than “rocks”.