A silver Half Crown dated 1677 - the reign of Charles 11.
Charles 11 reigned from 1660-1685.
A Half crown was 2 shillings & six pence.
By repute this coin was recovered from the wreck of the HMS Association but we have no certificate to prove this.
The coin reads 'CAROLUS 11'
It was common practice in the past to use Latin on British coins, especially when referring to the monarch's name. 
Measures 3.3cm diameter.