I have confirmed it is possible to solo a ruby statue using no gunpowder barrels. You need to find one,in shallow enough water you can just stand there. This way you can repeatedly attack with the cutlass without fear of shark attack or drowning. I also feel i got pretty lucky with no skellies poppin on me.
It seems the damage accrues over time and it just doesnt quite heal fast enough to survive the cutlass strikes.
Admittedly it took lile a solid 5 minutes and my finger got real tired lol. So i wont be continuing to do it but it was a fun challenge.
Cheers!