Before you go to your destination anywhere in the world, go to Google earth from there you can see what sort of coast line it will be. Before getting there, it will help you not wasting time to find a good place.
In sandy area like the beach or a cove, estuary and Mangrove flats you can find shells crawling or just buried under.
You can snorkel in sandy areas better done when the tide is low like that you don’t have to go so deep and it is easy to follow the track in the sand one or the other end you will found an Olive, a Natica, a Conus or a Terebra. I use to put some bait in the sand and with the smell dispersing in the water. You will see the Nassarius coming off the sand and Olive, too. As they are fast crawlers you don’t have to wait too long otherwise you can leave over night but make sure you secure the bait so the fishes won’t eat it. Then in the morning you can search in the sand all around where you put the bait you will find few shell in close vicinity.
In rocky areas and reef areas there are good places on or under rocks or dead corals but if you over turn rocks or dead coral make sure you turn back it in the same place so you don’t kill off every living thing that lives on and under the rocks.
You can collect shells from Trawlers if they are willing to take you on board. It is very rewarding most poor country will take you as long as you pay them and it is fare.
Some boat don’t dredge or trawl and you need one, you can make a trawling net or buy one to bring along with the fishermen.
You can visit fishing harbour and ask the fishermen to keep the shells for you if they are not allowed to take people on board,also you can look in the pill of rubbish that the fishermen caught in theirs nets,I found lost of shells species that I got from old tyres,inside bottles,old fishing nets etcetera.