not clear on offset

So if the real depth say via a lead line was 8 feet. And your transducer was 1 foot below the water line. The depth meter with no offset would read 7 feet. If your keel is 5 feet long below the hull (where the transducer also sits) and you put -5 feet as your offset then the depth meter would read 7-5=2 feet. In reality you should give yourself a little extra offset for safety this depends on your operating waters. In the Bahamas say you couldn't do an extra 5 feet because it would aways say you're aground. You'd do like maybe 1 foot extra i.e. a -6ft offset. Essentially you want to set your offset to the keel depth plus a bit. Yacht charter companies typically give it an extra 5 feet but you really don't know. Plus you don't know the last person who was on the boat maybe mucked with the offset and set a positive number. That's really bad. If they set positive 5 you'd run aground when the depth meter was saying you still had 9 feet of depth left. When I charter a boat, at the very first anchoring I go for a swim and check the depth below the keel vs what the meter says. Especially if you will be operating in shallow waters like Belize or Bahamas.