Thunderbrd
C2C War Dog
It'd also be in quite a few units in modules. Good to know that's the tag though.Indeed. One example is Civ4UnitInfos.xml, line 31134 - 31167:
Spoiler Terrain modification for the Galleass :
<TerrainImpassableTypes>
<TerrainType>TERRAIN_OCEAN</TerrainType>
<TerrainType>TERRAIN_OCEAN_POLAR</TerrainType>
<TerrainType>TERRAIN_OCEAN_TROPICAL</TerrainType>
<TerrainType>TERRAIN_TRENCH</TerrainType>
<TerrainType>TERRAIN_TRENCH_TROPICAL</TerrainType>
<TerrainType>TERRAIN_TRENCH_POLAR</TerrainType>
</TerrainImpassableTypes>
<TerrainPassableTechs>
<TerrainPassableTech>
<TerrainType>TERRAIN_OCEAN</TerrainType>
<PassableTech>TECH_ASTRONOMY</PassableTech>
</TerrainPassableTech>
<TerrainPassableTech>
<TerrainType>TERRAIN_OCEAN_POLAR</TerrainType>
<PassableTech>TECH_ASTRONOMY</PassableTech>
</TerrainPassableTech>
<TerrainPassableTech>
<TerrainType>TERRAIN_OCEAN_TROPICAL</TerrainType>
<PassableTech>TECH_ASTRONOMY</PassableTech>
</TerrainPassableTech>
<TerrainPassableTech>
<TerrainType>TERRAIN_TRENCH</TerrainType>
<PassableTech>TECH_ASTRONOMY</PassableTech>
</TerrainPassableTech>
<TerrainPassableTech>
<TerrainType>TERRAIN_TRENCH_TROPICAL</TerrainType>
<PassableTech>TECH_ASTRONOMY</PassableTech>
</TerrainPassableTech>
<TerrainPassableTech>
<TerrainType>TERRAIN_TRENCH_POLAR</TerrainType>
<PassableTech>TECH_ASTRONOMY</PassableTech>
</TerrainPassableTech>
</TerrainPassableTechs>
Edit: If you want to change all of these tags, grepWin is your friend. Just look for the string "<PassableTech>TECH_ASTRONOMY</PassableTech>", and for your own version replace it with "<PassableTech>TECH_SEAFARING</PassableTech>".
This tag is only in Civ4UnitInfos.xml (25 times), so this should be an easy replacement.