1 Long Tom: Be settin' course for London! Wi' an under-water boat we be able to be sneakin' through the Traitor's Gate and into the Tower o' London!
2 Long Tom: The Crown Jewels be bein' ours!
3 Wendy: Ye be wantin' to be breakin' in to the most feared prison in all Christendom?
4 Long Tom: Aye!
4 Wendy: I guess it be savin' the Admiralty some time.

I had to check that the pirates would indeed expect the Crown Jewels to be in the Tower of London. Thankfully, they've been housed there since 1303, so even 17th century characters should know they're in the Tower.

Although... the pirates are from the mid-1600s, which corresponds to the Civil War period in England, during which the original Crown Jewels were ransacked from the Tower, pawned, and/or melted down by Oliver Cromwell and his republican Commonwealth supporters. Most of them were irrevocably lost in this period, and new ones had to be made after the Restoration of the monarchy in 1660.

Presumably they haven't heard the latest news.