The trick with salvage, according to an old US Navy Salvage Master, is to get the water out and keep enough of it out to attain positive buoyancy - lots of ways to do that, provided the structure to be raised has the integral strength to withstand the forces required.

In the Mary Ellen Carter's case, "Thank God she's only sixty feet and the currents here are slow".

Cox and Danks raised the scuttled ships of the German High Seas Fleet from the bottom of Scapa Flow by cutting into the hulls and "winning" them back compartment by compartment. Cofferdams were attached to the hulls and built up in sections until the top of the coffer dam reached above the surface. The lowest section attached to the hull was an air lock. The book written about it, "The Man who bought a Navy" is a great read for those interested in that sort of thing.

On the radar assisted collision, even with the earliest radar sets (all relative plotting), three "paints" of any target will give you its actual range, relative course and relative speed from which its actual speed, actual course and CPA (closest point of approach) can be very easily calculated. As stated by Gareth above if those three "paints" show no indication of relative bearing change - unless you alter course/speed you are going to collide with that contact.