I have visited this country several times on expedition ships. However, due it's isolation and exposure to swell, we haven't always landed.
Niue is a tiny country in the South Pacific. There is only some 1000 or so people living on the main island. It is comprised of steep limestone cliffs.
On the island there are just a couple of beaches. There are several small bays in the limestone walls that are nice places to swim. There is also a Cheesfruit/Morinda/Noni farm. Most places require transport to get to.
The last time I was here in 2015, conditions were the best I have ever seen, and we had the chance to snorkel on the outside of the island. On either side of the main jetty, the coral drops off and is composed of great gullies and bommies. The fish life is diverse and colourful,
and includes larger life such as reef sharks and rays...
and of course various invertebrates..
The most famous aspect of this reef are the number of sea kraits. If you stand on the main cement jetty near town, you will see kraits a couple of times an hour at least. If you snorkel for an hour or two, you should see at least a couple of sea kraits. These are slimmer than true sea snakes, and may be inquisitive, but they are not aggressive.