ASSUMING that the information is correctly inputted, our GPS unit does show the vessel's length and relative position of the antenna. A Type A unit (which most ships and large vessels use) SHOULD take that into account.

Having said that, I've seen a number of large vessels whose information is obviously wrong -- starting with "destination". (Like listing Halifax as it's destination while entering the Delaware Bay from the north.)

The easiest mistake is when you see a tug, sometimes identifiable by its name. Example: Say, with 25' beam and 100' length. It doesn't show anything for the barge towing 200' behind it a 250' barge. Never get between a barge and its tow! Or a tug that's pushing a 200' long barge in front of it.

Radar might help when you see multiple images -- but that depends on range, bearing, size of screen, etc. Multiple sources of information are useful --- if they are comprehensible and taken with a grain of skepticism as to their reliability. When in doubt, NEVER cross in front of anything that is moving.

The discussions over the VHF are critical if you're in tight quarters.

As Doug pointed out, upon reflections, you want to i) make contact, ii) make them aware of you and confirm that they "see" you, iii) understand your mutual intentions, and iv) verify what both you and they will be doing.

I've had large a number of ships that have adjusted their course to pass clear of us when sailing. In other cases, they have said that they need swing room in tight quarters (e.g. where a narrower channel turns and they have to swing around). There are instances where you just don't get a response or get one that isn't comforting -- then you "just got to do, what you got to do."