We've snorkelled at many spots on Upolu, and this is by far the best. Located virtually right in Apia town at Vaiala Beach, it is an easy walk from Aggies Hotel and, if you're driving, offers safe and shady parking. There is an a 4Tala entry fee, and you can hire snorkelling gear, and use toilets/changing rooms.
To reach the actual reserve involves about 100 metres of snorkelling in fairly shallow water to a maker post. Don't stop right there! Another few metres takes you to an amazing drop-off, and wonderful viewing for hours. It's best to turn right and follow the edge of the drop-off in an anticlockwise direction.