Is it possible to have a cruise for 6 peoples in march . We want a 2 or 3 days cruise only.
No cruise line will have this, but you may be able to charter a small ship/boat/yacht for your adventure.
There is a Ferry or shuttle service between Ft Myers and Key West which takes about 4 hours.
I do not know of a "cruise" between the two.
Here are a couple websites for them.
That would be a rather short cruise. There are 3,4 day cruises from Tampa that visit key west and cozumel. There is a 4 hour cruise from Ft Myers to Key West for about $ 120 woth checking out. Its called "key West Express"