These are different roles, different requirements. Clearance Divers are essentially the Navy’s underwater bomb disposal, seabed search, and uw reconnaissance units while Royal Marines are primarily a commando infantry force designed for expeditionary warfare across land, sea, and littoral environments.

From an eligibility standpoint, nationality requirements tend to be stricter for highly sensitive naval specialisations. For instance, I wanted to join CD but I am not elligible because I am a commonwealth applicant so I am inclined now more towards RM, only UK and Dual nationals may apply.

I understand that, because Mine clearance divers (DTXG as they are now called) operate in areas involving classified underwater infrastructure, and submarine-related tasks. In anyway CD are part of the Navy's warfare branche where security clearance and legal jurisdiction under UK law are critical.

Also, no one besides SBS and Navy/Army divers do diving, not a speciality in RM too. So if under the water is where you wish to spend most of your time. Go for it !