From Public Policy Polling:
Bush continues to be well liked in the state, sporting a 51/40 favorability. Bush is universally popular with the Republican base, is unusually popular with Democrats at a 29% favorability, and is on positive ground with independents as well. In a hypothetical contest with Nelson he leads 49-44, taking a pretty substantial 23% of the Democratic vote and holding Nelson to a four point advantage with independents in contrast to the wide margins he has over the other contenders. [...] It's 44-36 over Connie Mack IV, 47-36 over George LeMieux, 44-32 over Mike Haridopolos, and 46-30 over Adam Hasner
Nelson is ahead against other contenders largely because they remain unknown to voters: Senator George Lemieux, appointed last year, remains unknown to 58% of voters, and 56% of voters say they don't know enough about Connie Mack. Bottom line: Bill Nelson should be worried even if Jeb Bush doesn't run.