HM Coastguard says crews were called out to the incident at about 16:35 BST on Sunday.
Luckily, a member of the public spotted the boys and called 999.
Cornwall Council says vandals targeted a public rescue buoyancy aid at Polzeath Beach.
Beach rangers say the juvenile female was in poor condition, with a swollen belly caused by liquid.
Rangers say the animal has been collected by Cornwall Wildlife Trust for a post-mortem examination
Beach ranger Andy Stewart shares advice after a car is stranded in a rising tide in Polzeath.
One man on board managed to get to shore from the yacht which is lying at an angle on the beach.
Anyone falling into the hole could have been seriously injured or suffocated, say beach rangers.
Crews were called to reports of the bin blaze at Polzeath beach car park on Tuesday.
