A pontoon based snorkeling area is about 400m from the beach and you need to book a shuttle by outboard boat to get there. I've forgotten how much it is. Also, there is a one hour limit although there's nothing to stop you buying another ticket to go back out but there could be a wait as the number in each group is limited. Snorkels, fins and mask are included or you can take your own. The pontoon is sited right on the edge of a huge drop over (volcanic crater rim). The water is 4m - 6m or more deep so there is no way you can stand on the coral, which is a problem at other islands that have been damaged by inconsiderate or ignorant tourists. I've snorkeled at many Pacific islands but none compare to this pristine site. IIRC there is a world record of 200 different species of sea life observed in a one hour period for this place. Note; there are NO private excursions available.
There is a short, pleasant, shady, walk to get to the beach from where the tenders offload and it is also the spot where other excursions come and go. It is a sandy track so those with mobility aids could have a problem. At this beach there is a well stocked bar, food outlets and souvenir shop.To buy anything you need to use island vouchers which can be purchased right there with Oz cash and possibly kina. It also a base for other tours such as catamaran sailing.
The beach is very nice and you can swim or snorkel there but the best is at the pontoon.
I didn't do any snorkeling in Alotau so I don't know what it was like. If there were wrecks it could have gotten my interest. The group I was with opted for a morning boat trip up and down the coast with an Australian educated but local WW2 expert giving a history talk on the Battle of Milne Bay. I'd recommend that for those with an interest in WW2 history. The afternoon was a visit to a family "farm" and got an insight on village life of the people there. There was cooking and tasting of produce from there gardens and cultural dancing. Very enjoyable. Both tours were ship booked (HAL).