March is a great time for swimming and snorkelling- the last of the long hot settled days of summer will run into April, especially in the top half of the North Island. Roberton Island out in the bay is perfect for all levels of snorkelling and also a great day to relax on the beach. Here's an independant boat trip you can take to get there-
Snorkelling is so-so around Pahia itself. Because Russell & Pahia both border(one either side) the lower reaches of a huge estuary and river system the seabed is quite muddy. It's also shingle on the Russell side. The best snorkelling is out around the many islands.
If you are going to be driving about a bit then Pahia is the better side to base yourself on otherwise you'll have to catch the car ferry twice a day(at $10+ a trip), and being restricted with timing. It runs between 6.30am & 9.30pm.
Do take a drive out on the Old Russell Road which follows the coastline out to Rawhiti and the open ocean and then down south. There are plenty of beautiful tiny bays to snorkel and potter about in.
The Cape Brett walkway is also out there and there are plenty of walks in and around the area.