1: The USCG only acts in a military capacity in certain situations. It is part of the Department of Homeland Security, and spends most of its time performing its maritime rescue and law enforcement missions.
2: You can definitely call the Coast Guard directly. Use channel 16 on a marine radio, which you'd probably have on a boat, which is when you'd need the Coast Guard.
I think the point the previous comment was that, if you are on land and you see and emergency out at sea, such as someone drowning! you can call 999 (and 112 and on a mobile also 911) and be put directly through to the coastguard as an added option, as well as using Channel 16 to contact the UK coastguard! which is a great advantage! countless life's have been saved this way in the UK!