The most common and most likely answer to this crossword clue is the 10 letter word WATERBORNE
I've seen this crossword clue in The Times
W
A
T
E
R
B
O
R
N
E
More clues leading to the result WATERBORNE
Buddy explains!

The straight part
We know that "on river" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on river = waterborne.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 WATERBORNE mean?
WATERBORNE adjective- Transported or transmitted by water
- Floating on the water; afloat