I just did a little diggin in the wiki and here is what i found.
"Valid spawn point
A spawn point is randomly selected between 40 - 80 meters of a random player in the zone. Spawn point is not valid if:

-Location has wrong biome
-The zone of the location has multiple biome (if full biome is required)
-Covered by a structure
-Blocked by player base
-Wrong altitude
-Wrong terrain angle
-Wrong amount of water
-A player exists within 40 meters
-Location is not inside a forest (if a forest is required). Forests are determined on the world generation. Terraforming or trees don't affect it."

And the only things that can spawn below -5meters of water are fish3 (everywhere) and serpents (ocean)

So if you dig 3 walls deeper that the water level nothing exept fish should spawn.
.... i should have done this an hour ago, sorry ;P.