Reminder for noobs:
Quinnipiac poll of Ohio Senate, 6/27/10:
Lee Fisher (D) - 42%
Rob Portman (R) - 40%
PPP poll of Ohio Senate, 6/26/10:
Lee Fisher (D) - 40%
Rob Portman (R) - 38%
Rick Scott is a well-known and somewhat popular figure in Florida so it shouldn't come as a surprise that, as of the present moment, FL-Sen appears to be a battleground but there's a case to be made that this state of affairs will evaporate after Labor Day once people start paying attention. In the end, Rick Scott has many liabilities, Bill Nelson is a popular incumbent and we're still in a Blue Wave environment.
Many of you are too young to remember the 2010 midterms or didn't follow politics at the time. I remember the polling breaking hard against Democrats when the campaigns kicked off in earnest. If you're a Republican, you shouldn't be optimistic about the midterms, even if there's plenty of reasons to be optimistic about Trump's odds of being re-elected.
edit: Florida sample is 43 R/39 D/15 I. This doesn't invalidate the poll, possible midterm electorate will look like this, but it's worth mentioning.
For sure, this poll (and other similar polls) do not mean that Nelson will lose. As you note, early polls are not particularly accurate, and Nelson will have to screw up big time to lose if, as expected, it is a D wave year.
But it is nonetheless concerning that we have at this point a slew of (low quality) polls, most of which show Scott slightly leading Nelson in FL. These things can both be true at the same time.
Yes, Scott is well known as the Governor, but Nelson should be well known as well, and supposedly he was/is popular (hence his previous easy re-elections).
What we should want to see is what we see in other states - Dem incumbents generally with leads, even in much redder states. Sure, what we see is partially a reflection of Scott's strength and not just Nelson's weakness, but Nelson really needs to step up his game.