My boat VHF has 25W and 5W capability. I tested it out for the first time this past weekend with a cousin of mine who lives on the lake. He was using his handheld in his livingroom. We were roughly 5 miles away from each other, I was obviously on the water..We were not in the line of sight with each other either (i.e. some islands in between us).
His handheld transmits at 5W. I could barely hear him, he was breaking up allot. He said that I was coming in clear as a bell..no issues. I also have an 8' antenna on the boat.