[Cainehill]

Here's the terrain types (had to use Google's Cache, since Shrapnel's search is busted - "terrain + 512" gives stuff that has no 512 anywhere in it, etc) :

nothing (defaults to plains) 0
small 1
large 2
sea 4
fresh water 8
mountain 16
swamp 32
waste 64
forest 128
farm 256
nostart 512 - for islands and special provinces
manysites 1024 - increased site frequency

Edit: So, I think you want to change terrain from 0 to 4, not 5, unless you want to make the province a small sea province (1 + 4). Medium would be 4, large would be 6.