longboard said..Anyone have ideas on how to get water out of the boom...?
End of my session cleaning up & I can hear water in the shaft sloshing back & forth...

OK, I'll hazard a guess for the Slick, having custom-made 3 carbon booms for the Echo.
The only real way to do this to ensure it doesn't happen again would be to drill out the rivets and pull out the plugs and redo the plugs. Either put in new plugs (Kids pool-kickboard material is good, I used a thin-walled pipe to push some plugs out) or you could silicone seal the old ones; I prefer the first option.
I also put some light Alu Screws with a fine thread through the curved front-piece of my Echo Booms (after I put good plugs in), so I could take them apart and redo the plugs from time to time (the clew end is glued shut with the end-plug so is watertight). You could do the same with the Slick front end; and try to seal the back end better if you think it leaks. I am not sure whether the Slick front end is letting water in somewhere, never seen one "live".
If you think it's just the rivets letting water in, let the thing dry out, silicone the rivets (both ends of the boom) and hope for the best next time... :-)
k.