Its all good advice Mwal. VHF is line of sight, so the higher, the better. However, depending on your boat size, you might find 8ft a bit awkward. I have a 3 foot stainless steel whip that came as a Cabela’s freebie when I purchased my Icom (M402-S). I have it mounted on the center console top grab rail which is 4 ft high, so I effectively have it about 7ft high. When its not in use, or I need to cover the boat, the mount swivels down and points the whip straight down the side of the console and out of the way. I can’t really tell you how far I can broadcast, I dont think I have ever tried more then 4 or 5 miles. I have never been out of range. The only other thing I would say is use good low loss coax cable and run it as short and direct to the radio as you can. I have been told that you can negate all the postive effects of a good antenna with a bad coax connection.
Good luck!
Dave