THE 798Csci unfortunately cannot handle a Platinum+ chip, so you'll want to look for a Gold 1XG chip (US & N. Bahamas all in one).
OR...if you don't need coastal coverage out beyond 3 mi. from shore, the Marine & Lakes product could be perfect for you. $99 gives you all freshwater for E. FL, plus coastal charts out to 3 miles. Includes all of Lake O and the Keys.