I think you need to watch it very carefully to be on the safe side. I don't know what kind of breed The Land Shark is, but since sharks are predatory creatures, you should be aware of something called predatory drift. This is when a dog is initially playing in a benign way, but when the partner does something that makes it seem a bit like prey, such as squeaking, can switch into predation mode. It's something that Ian Dunbar talks about; anyway you can read more about it here for a fuller explanation.
Some breeds are less predatory than others, so for example if your Land Shark was a siberian husky I would be very worried, since huskies have a strong prey drive; on the other hand, some breeds of dogs wouldn't really know what to do if they actually caught something.
However as well as protecting the cat you also have to think of potentially protecting the shark from a claw to the nose, which would be very painful.
In any case, it is always useful to be able to call a dog out of such situations, so you might want to practise calling the Land Shark out and then letting him return to play if you wish, or else giving him some other big reward. This will make it easier to get him to stop suddenly if you ever need to for some reason.
Even when a dog and cat get on remarkably well, the relationship can change very suddenly e.g. if a cat runs and prey drive is triggered.