Things that determine the rate of loot spawning:
1) Whether you are diving and have the goggles.
2) Areas with a lot of water lowers the chance slightly(such as sitting in the York Town Bay)
3) Having already obtained an item in that surrounding area slightly decreases the rate
4) Being in a new area increases the rate
5) Spawn spots already existing decreases the rate slightly
6) The more to items you have found in a specific area, the less likely you are to find more items in that area.
All in all, diving was designed to encourage movement, and it was designed to punish those who sat in one spot looting over and over. To increase your chances the best thing you can do is create a route in which you loot a specific area for a little while, then move on to the next area and repeat.