The most common and most likely answer to this crossword clue is the 5 letter word RAFTS
I've seen this crossword clue in Financial Times
R
A
F
T
S
Buddy explains!

The straight part
We know that "they float" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they float = rafts.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does RAFTS mean?
RAFTS noun- A flat-bottomed craft able to float and drift on water, used for transport or as a waterborne platform.
- (by extension) Any flattish thing, usually wooden, used in a similar fashion.
- A thick crowd of seabirds or sea mammals, particularly a group of penguins when in the water.
- A collection of logs, fallen trees, etc. which obstructs navigation in a river.
- (when ordering food) A slice of toast.
- A square array of sensors forming part of a large telescope.
- To convey on a raft.
- To make into a raft.
- To travel by raft.
- To dock (toolbars, etc.) so that they share horizontal or vertical space.
- A large (but unspecified) number, a lot.