A maximum of two ships are allowed per sea tile, island or smugglers cove. Only in the two harbors (The Orient and The Republic) more than two ships are allowed, but these places are save from combat. Hope this helps.
This is incorrect.
In the rulebook in the chapter of the gameboard (English page 10) the first paragraph it states:
A maximum of two player ships is allowed per tile, except on the Republic, the Orient and on Smuggler's Cove, these have no limit.
You cannot attack more ships when you enter the cove. In the rulebook (page 6) it states: you start combat by using your captain card (turning it face down), if you captain is already face-down you cannot attack....
This means that after you have attacked one other ship, you're captain is turned face-down.
You can be attacked by all other players. That is also mentioned in the same paragraph.