Cruz Bay is the western bay and town on St John. It is important because it is the easiest place to check into the US. When you are entering remember there are two channels. The one on the right is for the St Thomas ferry while the channel on the left is for the BVI ferries and anyone else who needs to check in. You can dock your boat at Customs, but you will get charged for it. Instead I anchor to the left of the channel where the National Parks Service (NPS) has an area big enough for 4-5 boats. You are only allowed 3 hours anchoring here, but it is free and plenty of time to check in and walk around town for a bit.
Again you can land your dinghy at the Customs building, but it will cost you. Instead go to the NPS dinghy dock to the left of Customs. While you are here you should check out the NPS visitors center and check out the displays about the island including a fantastic 3D map of the island (1st and 2nd photo).
After you are done with the NPS and Customs you can check out the shopping, which is locally made and quite artistic from jewelry to souvenirs. Right down the street you can also see the battery, which use to protect the harbor (3rd photo). Unfortunately the museum is not up and running at this time, but there is some history here.
If you start walking up the hill you will find Dolphin Market and St John Ice Company for convenient provisioning and the cheapest ice I have found in the VI. While here you may want to visit the Elaine Sprauve Library. This is a formal great house (think small scale plantation house) and has been very nicely restored. Plus there is a small garden next to the family cemetery all of which is worth a visit due to its beauty.
So to conclude Cruz Bay has more to offer than checking into the USA. You have unique shopping you won’t find on St Thomas, some historic sites, easy provisioning, and a gateway to explore the rest of St John by hiking trail or rental car if you choose.