I haven't seen the first one, but I read this sequel was only tangentially connected. And sure enough, this is easy enough to watch as a stand-alone blockbuster. It's a fairly typical Johnson flick, mixing comedy and adventure with questionable CG. The result isn't anything special, but it's still quite entertaining.
When Sean finds an encrypted message, his stepdad helps him to crack the code. The message reveals the hidden location of the Mysterious Island described in Jules Verne's book. Together they take a trip to Palau to try and find the island, but the place is way more dangerous than they expected.
Dwayne Johnson is made for films like this, the film is quite self-aware, with the comedy making good use of that and the short runtime turned out to be a blessing. The CG is quite horrendous though and Peyton's suitable-for-all-ages directing sensibilities aren't that great, but other than that, this was decent entertainment.