Malo lava SmudgeNZ
There really is nowhere that you can either swim or snorkel for 'free' in Samoa. The fee you pay to use a beach is sometimes the only source of income many villagers have, the beaches are actually on their cultural land so you pay to swim on them. This can be as little as 10 tala up to 25 tala depending on the beach you are on.
The guys at Aqua Samoa Watersports, which is based at Aggies, will point you in the right direction without necessarily trying to sell you anything. There are many beaches in Samoa where the snorkelling is great, both on Upolu and Savaii. Lalomanu is probably one of the best and you can either do a tour there (which includes a lenghty stop for snorkelling and lunch) or hire a car and drive there (around 100 tala a day), and then pay 20 tala each to park there and swim/dive on the beach, but in the long run works out cheaper than a tour if you're going to use the cars over a couple of days. There is really nowhere close by to Aggies that I would call great for snorkelling, except the reef and you would have to get someone from Aqua Samoa to take you out or hire a boat. You can email them on info@aquasamoa-watersports.com or phone +68545662.
I think your best bet is to hire a car and then get yourself around to the south coast, Lalomanu, the coves around Matereva and in the east along the Aleipata coast. There is also great snorkelling at Palolo Deep Marine Reserve in Apia.
You could also consider staying at one of the resorts on the south coast - Sinalei, Coconuts or Virgin Cove and then you would be much closer to the best snorkelling spots.
If you find a deserted beach with no-one around, go for it, but if you are at a beach in a village you should always ask permission to use the beach and that's where the fee comes in.
I hope this helps. If you decide to hire a car just let me know and I'll give you some contacts. Any further questions please ask.
Fa soifua
Deb