A straightforward day visit off a cruise ship is generally covered by the Falklands' in-transit exemption. The government states that a visa national who is a passenger or crew member in transit doesn't need a visa when transit is the principal reason for landing, and its own guidance specifically names day cruise ship passengers. That covers travellers whose nationality would otherwise need a visitor visa.
Don't stretch that to every itinerary. It should not be assumed to cover an overnight stay before or after the cruise, a land-based programme, an independent arrival by air, or a voyage that begins or ends in the Falklands. Those need a nationality-specific check with Falkland Islands Customs and Immigration.
On passports, the visitor information page says validity through the length of your stay, while Customs and Immigration separately recommends at least 6 months remaining at entry. Airlines and onward countries can be stricter, so 6 months is the safe position.
If your itinerary isn't a simple day call, contact Falkland Islands Customs and Immigration before you book flights or accommodation.