Drill a little hole in it????

From memory it is a one piece so the water has either got in past the plugs at the extension end or it's cracked.
Check the extension because they often fill with water also.
If it's cracked you should hear it creaking or rattling when you jar it.
Most likely the plugs though!
Remove the extension, lean the boom on something at about 45 deg in the hot sun with the head to the top.
When it heats up the pressure inside should push the water past the plugs.
Do Not Drill Holes!!!!!!!!!
Worst case you will have to remove one of the plugs, difficult job.

Paul

reefer, there will be a u-shaped plastic plug at the end of the extension and you need to push a long piece of sturdy fencing wire with a small hook bent into it, up there to push the plug sideways and then pull it back out. (might need a smack or two with a hammer to dislodge it!)
then drain water (usually smells like a fish morgue), flush with fresh and replace plug with a bit of silicone on top of it.