I don't know about salt water. I have a '79 Venture 25 and inspected the keel bolt a few years ago. It showed no signs of wear so I put it back in.
I'm assuming you have an older boat with a cast iron keel like mine. To inspect/replace you need to get the boat up off the trailer enough to be able to support the keel in the front and back. It needs to be supported in the back to keep from swinging fore and aft slightly after the bolt is out so that you don't lose track of the hole alignment. In the front support it wit a jack. Put a little upward pressure on the jack to take the weight off the bolt then you can remove the bolt and replace it.
I got my boat up off the trailer by letting the trailer jack all the way down which raises the stearn. Then putting something under the stearn that would hold the back of the boat. I built an oversized A frame type saw horse to fit under it. The screw the trailer jack all of the way up. That gets the rear off the trailer. Then, I used some blocks and put a bottle hydraulic jack under the front and slowly raised the front. I had some 2X4's handy that were long enough to span across the rails of the trailer. As I raised the front of the hull to make enough room I slid a 2X4 under so that the hull (in front of the keel) sat on a stack of 2 - 4 2X4's which were supported by the side rails of the trailer. Just go slowly and have the next board ready to go so that you aren't raising too high for too long balanced on the jack.
Then I could get the jack under the front of the keel to take the weight off the bolt.
Hope that helps,