What is this? What it used for and do I need it......Radio thing? Regulator?
(http://i1220.photobucket.com/albums/dd442/Staseadawn/IMG_1704.jpg)
What you're looking at is a heat sink for something that converts/controls electrical power. The wire color (green with yellow) suggests a ground, but if your boat has been as badly maintained as mine, the wire color is meaningless.
Based on where it's mounted, it's probably not anything to do with radios. Where do the green/yellow wires shown go to? Are they the only wires? (doubtful). If they are, it's some kind of in-line filter (wires are same, so no polarity, it's not a rectifier, possibly a power conditioner to make the DC quiet) Of course someone may have replaced the wires with something convenient, so a red and black become a green and green.
Probably a battery isolator (if at least 3 connections), but it could be a regulator( needs input, ground and output), an inverter (DC in, AC out) or a charger (AC in DC out). It's function can be divined by looking at where the wires that connect it go to. If you have one side going to AC and the other to the batteries, it's either a charger or an inverter. Usually a charger will allow you to adjust the float voltage, so you would want to take that panel off to look for adjustment pots. If it goes between the alternator and the batteries, it's either an isolator or a regulator. Just like the charger, the regulator may have an adjustment, or not.
A regulator (voltage regulator) will have an input from the alternator, and a single output (two identical wire suggest this is not the case). An isolator will have an input from the alternator and may have multiple (1,2,3 or 4) outputs that go to the batteries.
Wow...thanks...I will do some investigation and repost