ten carat hill underwater cave??

this might be such a silly question but its my first time playing Pokemon Moon and I saw I could catch Psyduck in Ten Carat Hill. i didnt find them in there, so googled and it says it’s in the underwater cave? I have SCOURED this area and seemingly cannot find any underwater cave.

(At the moment, I can’t break the huge stone brick at the end of the cave (next to the hollow), is it past there or am I just not seeing something?)

Thanks :) !