When you do an excursion do you book through the ship or on your own? I have found that most excursions can be bought in port on your own for sometimes half the price the ship charges. So far never missed the ship, but the ship has had to wait for us a few times because their excursions were late.
We've always bought from the Cruiseline for the very reason you mentioned - if the ship's excursion is late, the ship won't sail away, but if a privately booked excursion comes back late your ship may have already left the port. Next month we are going with a tour booked through Cruiseline.com because they have a guarantee of getting us back in time, and it's due back to the port a couple of hours pre sailing. On some islands, there's no way I'd take a chance after seeing some of the vehicles they have. The extra price is well worth the peace of mind. Also, Carnival has a guarantee about the price for the exact same excursion.
In addition to searching previous posts on this topic, you might want to consider the following when it comes to excursions. Usually the ship only waits for excursions purchased via them and its only been recently that some companies make that guarantee as well.
The port of call and where it is determines what I do on shore. For example, I only did excursions via the ship for Alaska because they involved seaplanes, helicopters and such and it was a once in a lifetime tour I wanted to be sure went well. I am so familiar with the ports in the Caribbean that I always "do my own thing" - be it rent a car, a tour guide, hop on hop off buses, etc. The Caribbean is like a second home to me and getting around is second nature. I never rent anything in Mexico and while I've done both ship excursions and local excursions, I tend to stick close to the port and maybe take a shuttle.
First time cruisers usually benefit from booking a shore excursion via the ship because they are also literally guided to where they need to be (e.g. gather as a group on the ship, disembark, and then led to their tour bus)
Frequently, I will get off the ship, wander a bit, scope out what's available and decide that day. I love talking to local joggers who have a plethora of information.
Repeat after me about excursions - The Captain Waits for No One!
I will usually get the ship's version over the private version:If the excursion runs late due to a problem, the ship will either wait for you or pay to get you to the next port. If there is a medical problem, the line assumes some responsibility (if not legally required, then they do it merely for PR reasons), rather than depending on a native local who probably couldn't afford to help you. If the ship doesn't stop at the port for whatever reason, with a ship excursion you are guaranteed a full refund. Not so with a private excursion.
For me it depends on if I know the port or not and how safe I think the port is. It is always nice to know that Excursions through the ship have some guarantees on Quality, safety and not being left behind if your bus has problems and your late to port.
Unless we are just doing independent sightseeing, we always book through the ship-sponsored ones for peace of mind. We don't want to miss the ship.